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TANYA'S

COMPREHENSIVE GUIDE TO

FELINE CHRONIC KIDNEY DISEASE

 

 

 

HYPERTHYROIDISM (OVERACTIVE THYROID)

 

ON THIS PAGE:


What is Hyperthyroidism?


What is Hypothyroidism?


Possible Risk Factors


Symptoms


Masking of CKD


Diagnosis


Treatments Overview


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Home > Related Diseases > Hyperthyroidism

 


Overview


  • Hyperthyroidism, or an overactive thyroid, is relatively common in older cats.

  • It is caused by a tumour, which in most cases is benign (non-cancerous).

  • Hyperthyroidism can cause a number of problems, including heart problems, so it is essential to treat it if present.

  • There are a number of possible treatments, all with pros and cons.

  • Once the hyperthyroidism is under control, CKD may be revealed (or, if already present, may appear to worsen) as true kidney function is revealed.


What Is the Thyroid?


 

The thyroid gland is a butterfly shaped gland, with two lobes. It is situated at the front of the neck.

 

The thyroid is responsible for the regulation of metabolism. Imbalances may sometimes occur as follows:

  • hyperthyroidism means the thyroid is overactive. This is relatively common in cats, especially in older cats. It is thought that over 10% of cats over the age of ten will develop hyperthyroidism.

  • hypothyroidism means the thyroid is underactive. This is very rare in cats.

  • If you struggle to remember which is which, the "po" in hypothyroidism sounds like low.

  • euthyroid means the thyroid is in balance.


Hyperthyroidism (Overactive Thyroid)


What Happens in Hyperthyroidism


The most common cause of hyperthyroidism is a tumour known as an adenoma, which is usually benign (i.e. the tumour is not cancerous). In most cases, both lobes of the thyroid gland are affected. The tumour usually grows over time, but the timeframe varies from cat to cat. Because the tumour increases the size of the thyroid gland, the gland produces more thyroid hormone than is needed by the body, i.e. it becomes overactive, which is referred to as hyperthyroid.

 

Since the thyroid controls metabolism, this overactivity causes all of the body's processes to speed up.

 

Hyperthyroidism often causes heart problems. Risks associated with radioiodine therapy for feline hyperthyroidism Advanced Veterinary Medical Imaging says "The chronic elevation of circulating thyroid hormone levels in cats with hyperthyroidism leads to changes in their heart muscle that predispose them to developing heart failure." Fortunately, heart problems caused by hyperthyroidism may be reversed if the hyperthyroidism is successfully treated, though some cats will require ongoing treatment for heart problems.

 

Hyperthyroidism: a view from the urinary tract (2010) Grauer GF CVC in Kansas City Proceedings says "Systemic hypertension [high blood pressure], proteinuria, and urinary tract infection (UTI) can be consequences of either hyperthyroidism or CKD." Proteinuria may improve once the hyperthyroidism is under control.

 

Untreated hyperthyroidism can also affect phosphorus and calcium levels and cause hyperparathyroidism.

 

Left untreated, hyperthyroidism will eventually be fatal. Fortunately it is treatable.

 

Long Beach Animal Hospital has a good overview of hyperthyroidism in cats.

 

International Cat Care has an overview of hyperthyroidism.

 

Pet Place has some information about hyperthyroidism.

 


Hypothyroidism (Underactive Thyroid)


 

Occasionally a cat may appear to have the opposite problem to hyperthyroidism, i.e. hypothyroidism. This means the thyroid is underactive, so metabolic processes are slower than they should be.

 

Hypothyroidism is common in dogs and humans but is extremely rare in cats. The two main causes of hypothyroidism in cats are as follows. One will require treatment and the other will not.

Non-Thyroidal Illness Syndrome


If bloodtests indicate that a cat with another disease, such as CKD, has low thyroid levels, it is more likely that the cat has "non-thyroidal illness syndrome" or NTIS (formerly known as "euthyroid sick syndrome" or ESS). The concomitant disease.makes T4 levels (used to diagnose thyroid problems) appear lower than they actually are.

 

NTIS does not usually need any treatment, because if you are able to bring the accompanying disease under control, the thyroid hormones should return to a normal level.

 

NTIS may also be caused by starvation, so if your cat has not been eating, this might be a factor.

 

Anaesthesia may also be a factor.

 

Regulation and significance of measured thyroid hormone levels in health and in disease Newman C discusses euthyroid sick syndrome.

 

Euthyroid sick syndrome (2017) Aytug S Medscape discusses euthyroid sick syndrome in humans but the principles are the same.

 

Following Treatment for Hyperthyroidism


Hypothyroidism may also appear after treatment for hyperthyroidism. Often this is mild and transient and will not need treatment, but some cats do need temporary or permanent treatment. See below for more information.

 


Possible Risk Factors


 

It may surprise you to learn that hyperthyroidism was unknown in cats until 1979. Therefore there is much speculation about possible risk factors that may increase the chances of a cat developing the benign tumour that causes hyperthyroidism. :

PBDEs (Flame Retardants)


Chemicals known as polybrominated diphenyl ethers (PBDEs) are commonly used as flame retardants. There appears to be a correlation between the increased use of these products in the home and the rise in feline hyperthyroidism. These products are thought to mimic thyroid hormones, and thus may be a factor in the development of hyperthyroidism. Flame retardant chemicals in house dust linked to  hyperthyroidism (2012) Peterson ME Insights into Veterinary Endocrinology says "In support of that hypothesis, major PBDE production began just before the time that hyperthyroidism was first recognized in 1979."

 

Elevated PBDE levels in pet cats: sentinels for humans? (2007) Dye JA, Venier M, Zhu L, Ward CR, Hites RA & Birnbaum LS Environmental Scientific Technology 41(18), pp6350-56 measured levels of PBDE in 23 cats, eleven of whom had hyperthyroidism. The cats with hyperthyroidism had three times as much PBDE in their bodies as the healthy cats. Indoor cats had particularly high levels, which may be because they are constantly exposed to these compounds within their homes. Tying in with the food study below, indoor cats who favoured fish-flavoured canned food appeared to be most at risk of developing hyperthyroidism; and it appears that these foods contain the highest levels of PBDE.

 

The feline thyroid gland: a model for endocrine disruption by PBDEs? (2012) Mensching DA, Slater M, Scott JW, Ferguson DC & Beasley VR Journal of Toxicology and Environmental Health Part A 75(4) pp201-12 found that total levels of PBDEs were not significantly different in household cats with and without hyperthyroidism but were lower in feral cats. The highest concentrations were found in household dust, and dust levels were much higher in the homes of hyperthyroid cats.

 

Association of polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs) with hyperthyroidism in domestic felines, sentinels for thyroid hormone disruption (2017) Walter KM, Lin Y, Kass PH & Puschner B BMC Veterinary Research 13(120) looked at the effects of these chemicals on cats and concludes "elevated exposure to both PBDE and PCB congeners is associated with FH, supporting the hypothesis that these persistent organic pollutants may contribute to the etiopathogenesis of feline hyperthyroidism," but goes on to say that further studies are needed.

 

Dietary Concerns


 

Canned Food


Epidemiologic study of relationships between consumption of commercial canned food and risk of hyperthyroidism in cats (2004) Edinboro CH, Scott-Moncrieff JC, Janovitz E, Thacker HL & Glickman LT Journal of the American Veterinary Medical Association 224 (6) pp 879-886 examined the records of just under 170,000 cats over a 20 year period. The study found that the consumption of canned food appeared to carry an increased risk (almost four times the risk compared to cats who ate dry food) of developing hyperthyroidism, and that the risk increased for every year that the cat ate canned food.

 

A later study, Risk factors for feline hyperthyroidism in the UK (2009) Everard A, Brodbelt D, Elliott J, Syme H Journal of Small Animal Practice 50 pp406-14 found that "exposure to food packaged in a can was identified as the major risk factor for the development of hyperthyroidism." It also found that non-pedigree cats, indoor cats and cats who used a litter tray were more likely to develop hyperthyroidism.

 

The reason for these findings is not known at this time, but it may possibly be related to the lining used in these cans, bisphenol-A (BPA), a potential endocrine disrupter. Males appeared to be at risk only if they ate food from pop-top cans, i.e. standard canned food did not appear to carry a risk. However, females appeared to be at increased risk whether they ate food from pop-top cans or other types of cans. 

 

Whilst these finding are a concern, generally speaking CKD cats do better on a wet food diet (see Which Foods to Feed). A possible solution might therefore be to feed food in foil pouches or plastic trays. If you do use cans, be sure to store any leftover food in glass containers in the fridge rather than in the cans themselves. The University of Warwick has more information on this.

 

Food Flavours


Evaluation of dietary and environmental risk factors for hyperthyroidism in cats (2000) Martin KM, Rossing MA, Ryland LM, DiGiacomo RF, Freitag WA Journal of the American Veterinary Medical Association 217(6) pp853-856, found that "cats that preferred fish or liver and giblets flavors of canned cat food had an increased risk". Fish-flavoured foods appear to contain the highest levels of PBDE (see above).

 

Soy


Effect of dietary soy on serum thyroid hormone concentrations in healthy adult cats (2004) White HL, Freeman LM, Mahony O, Grahan PA, Hao Q & Court MH American Journal of Veterinary Research 65(5) pp586-91 found that soy in a healthy cat's diet may increase T4 levels.

 


Symptoms


 

The classic symptom of hyperthyroidism in cats is that the cat loses weight despite having a good appetite and eating a lot.

 

Hypertension (high blood pressure) is extremely common in hyperthyroid cats. Hypertension in cats with chronic renal failure or hyperthyroidism (1990) Kobayashi DL, Peterson ME, Graves TK, Lesser M & Nichols CE Journal of Veterinary Internal Medicine 4 pp58-62 found that 87% of the hyperthyroid cats in the study had hypertension.

 

If your cat has CKD, you may not necessarily suspect that your cat has hyperthyroidism, because some of the other symptoms of hyperthyroidism are also common in CKD cats, including:

  • increased urination

  • increased thirst

  • a greasy coat

  • spiky fur

  • vomiting

  • hoarseness

  • restlessness

  • howling (especially at night)

  • a fast heart rate (tachycardia)

  • sometimes a heart murmur

  • scratching a lot and pulling out fur. Newman Veterinary has a photo of the result of such behaviour.

One major difference is that CKD cats tend to lose weight and their appetites, whereas hyperthyroid cats tend to lose weight despite having a good appetite.

 

A small number of hyperthyroid cats have a form of hyperthyroidism known as "apathetic hyperthyroidism" and become lethargic and lose their appetite instead. This only happens in a small number (less than 5%) of cats, but these cats are more likely to have CKD or heart disease as well as hyperthyroidism.

 

Top 10 signs of hyperthyroidism in cats (2011) Peterson ME Insights into Veterinary Endocrinology discusses the main signs of hyperthyroidism in cats.

 

Pet Education has a helpful article about hyperthyroidism which gives the. likelihood of the various symptoms.

 


Masking of (Hiding) CKD


 

Unfortunately, as well has having some similar symptoms to CKD, hyperthyroidism can actually hide the presence of CKD or make CKD appear to be less severe than it actually is.

 

This happens for a number of reasons:

  • One of the body's metabolic processes which is speeded up by hyperthyroidism is the glomerular filtration rate (GFR), a measure of kidney function. Increased GFR can lead to reduced BUN and creatinine levels.

  • Hyperthyroidism also increases bloodflow through the kidneys, which makes them appear to be working better.

  • The loss of muscle mass commonly associated with hyperthyroidism may also reduce creatinine levels (since creatinine is a by-product of muscle).

Therefore it can seem as if the kidneys are functioning well even in cases where they are not. In other words, hyperthyroidism does not cause CKD, but it can mask (hide) the presence of CKD, or make it look less severe than it is.

 

Since over 10% of cats over the age of ten have hyperthyroidism and around 10% of cats over the age of ten have CKD, it is inevitable that some of these cats are actually suffering from both conditions; but in such cats, the true condition of the kidneys only becomes apparent once the hyperthyroidism is treated, and it is often at this point that CKD is finally diagnosed.

 

This does not mean that hyperthyroidism should not be treated; on the contrary, it is essential that hyperthyroidism be treated. What it does mean is that you need to do it very carefully in a cat with confirmed or suspected CKD. See below for more information on this.

 

Feline hyperthyroidism and its relation with renal function (2006) Daminet S Presentation to the 31st World Small Animal Veterinary Association World Congress discusses how hyperthyroidism affects the kidneys.

 

Complications and consequences of feline hyperthyroidism (2003) Mooney CT Presentation to the 28th World Small Animal Veterinary Association World Congress which explains more about the effects of hyperthyroidism on the body.

 


Diagnosis


 

It can sometimes be difficult to diagnose hyperthyroidism because many of the common symptoms can be seen in other diseases (e.g. increased drinking may be seen in cats with diabetes or CKD), and some cats have borderline hyperthyroidism, so the usual tests may not provide a definitive diagnosis. The vet therefore needs to look at the overall picture.

 

2016 AAFP guidelines for the management of feline hyperthyroidism (2016) Carney HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR Journal of Feline Medicine & Surgery 18(5) pp400-416 state "Diagnosis requires the demonstration of persistently elevated thyroid hormone concentrations (T4, or T4 plus fT4ed) occurring concurrently with one or more of the typical clinical signs."

 

The guidelines for diagnosis and treatment may be summarised as follows:

 

Category Symptoms Present T4 Thyroid Gland Suggested Response

Group 1:

Classical clinical disease

Yes Elevated Enlarged Treat for hyperthyroidism.

Group 2:

Possible hyperthyroidism with probable non-thyroidal disease (e.g. CKD)

Yes Normal   Repeat T4 test, together with fT4ed test, after 2–4 weeks.

Evaluate for other diseases.

Consider T3 suppression or thyroid scintigraphy

Group 3:

Enlarged thyroid without clinical hyperthyroidism

No Normal Enlarged Monitor symptoms.

Repeat T4 test

in 6 months

Group 4:

Subclinical hyperthyroidism

No obvious clinical signs but some symptoms suggestive of hyperthyroidism Elevated   Repeat T4 test in 2 weeks, treat if elevated.

If normal, repeat T4 test in 6 months

Group 5:

Clinical hyperthyroidism with confirmed non-thyroidal illness

Yes Elevated   Treat for hyperthyroidism.

Manage other diseases as appropriate.

Group 6:

Clinically normal

No Elevated No Repeat T4 test.

Treat if elevated.

If normal, monitor symptoms and repeat T4 test in 6 months

 

Here is some information on the tests mentioned in the table which are used to diagnose hyperthyroidism:


Physical Examination


 

Your vet will take your cat's history and ask you about possible symptoms of hyperthyroidism. Your vet will also physically examine your cat.

 

The vet can often (but not always) feel a nodule in the thyroid. 2016 AAFP guidelines for the management of feline hyperthyroidism (2016) Carney HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR Journal of Feline Medicine & Surgery 18(5) pp400-416 states "Palpably enlarged thyroid glands are suggestive, but not necessarily indicative, of clinical hyperthyroidism."

 

The vet may also detect high blood pressure and/or a faster heart rate than usual, while the heart may be enlarged. 

 

One vet told me he has found that if he pinches a cat's ear, its pulse will slow down, except in hyperthyroid cats. Obviously, this is not a proper method of diagnosis, but you may want to check this occasionally at home.

 


Blood Tests


 


Blood Chemistry and Complete Blood Count


Apart from specific blood tests for hyperthyroidism (see below), routine blood tests are usually run, which may show the following:

SDMA in hyperthyroid cats before and after treatment with radioiodine therapy is a study at Virginia-Maryland College of Veterinary Medicine to see if the new SDMA test "can help identify cats with underlying kidney disease and improve treatment outcomes."

 


Thyroid Blood Tests


 

Additional blood tests to check thyroid function will also be run, which often need to be sent to an external laboratory for analysis.

 

Michigan State University has a helpful overview of the significance of the various tests.

 

Current concepts on diagnosing and managing thyroid disease in dogs and cats: a roundtable discussion (2010) Robertson J, Nelson R, Scott-Moncrieff JC & Kintzer P NAVC Clinician's Brief Nov 2010 discusses the diagnosis and treatment of hyperthyroidism,

 

T4 or Total T4


The most widely used blood test for thyroid function in cats is the T4 test, also known as Total T4. This measures levels of a hormone called thyroxine. What is T4? How is the T4 test used to diagnose hyperthyroidism in my cat? (2010) Peterson ME Insights into Veterinary Endocrinology explains more about T4.

 

Some vets can run this test in house, but many prefer to have it run by an external laboratory. Comparison of in-clinic point-of-care and reference laboratory total thyroxine immunoassays for diagnosis and post-treatment monitoring of hyperthyroid cats (2017) Peterson ME, Rishniw M, Bilbrough GE & Cote KB Journal of Feline Medicine & Surgery 2017 May epub ahead of print compared the IDEXX Laboratories Catalyst Total T4 in-house test with one performed at a laboratory and found that "the in house analysis of TT4 provided reliable and accurate values for TT4, comparable to the reference lab. It appears that the in house testing is largely equivalent to the reference lab for classification of cats as hypo- eu- or hyper- thyroid, however borderline cases may need to be re-tested or confirmed by other methods."

 

Here is a rough guide to T4 levels but the laboratory used by your vet will provide its own reference range, which you should rely on:

 

T4 Level: US Values T4 Level: International Values Interpretation
Below 1.0 ug/dl Below 13 nmol/L Low (Below normal)
1.0 - 5.0. ug/dl 13 - 64 nmol/L Normal
2.5 - 5.0 ug/dl 32 - 64 nmol/L Grey zone
Over 5.0 ug/dl Over 64 nmol/L Consistent with hyperthyroidism

 

T4: High


A T4 reading over around 5 ug/dl or 64 nmol/L indicates that hyperthyroidism is probably present, but this must be considered in conjunction with other findings, because in a small number of cats (less than 10%), the T4 can be elevated even if the cat does not have hyperthyroidism.

 

What steps should you take if your cat's T4 level is elevated? According to 2016 AAFP guidelines for the management of feline hyperthyroidism (2016) Carney HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR Journal of Feline Medicine & Surgery 18(5) pp400-416:

  • If T4 is elevated, the thyroid is enlarged, and there are clinical signs of hyperthyroidism, you should treat for hyperthyroidism.

  • If T4 is elevated, there are clinical signs of hyperthyroidism and your cat has another disease (e.g. CKD), you should treat for hyperthyroidism and also treat the other disease. See below for more information on managing both hyperthyroidism and CKD.

  • If T4 is elevated but there are no real signs of hyperthyroidism and no enlarged thyroid, you should repeat the T4 test in 2 weeks, and treat for hyperthyroidism if it remains elevated. If, however, the second T4 test is normal, you should repeat the T4 test in 6 months.

T4: Grey Zone


The table above contains a grey zone area, which as you can see is still within the normal range, but cats with T4 in the grey zone need to be looked at more closely. This is because the normal ranges do not apply to older cats (over the age of 10), so older cats with a T4 reading in the grey zone may nevertheless have hyperthyroidism. A grey zone reading may also not be accurate in cats with other illnesses, such as CKD.

 

Frequently asked questions: thyroid function in cats Michigan State University Diagnostic Center for Population and Animal Health says "Most hyperthyroid cats have high serum concentrations of thyroid hormones. However, there are a small proportion of hyperthyroid cats which have normal or only slightly elevated thyroid hormone concentrations. This phenomenon is termed "occult hyperthyroidism." A number of possible explanations exist for this phenomenon including the presence of thyroid pathology which is not yet sufficiently severe to be easily documented (mild disease); the presence of a concurrent non-thyroidal illness; and day-to-day or week-to-week fluctuations in thyroid hormone concentrations in hyperthyroid cats which sometimes dip into the reference range."

 

In fact, about 10% of cats in the grey zone will have hyperthyroidism despite having a theoretically normal T4 level. Evaluating another thyroid function test for hyperthyroidism in cats (2015) Winn Feline Foundation says "Approximately 10% or less of hyperthyroid cats, and over 30% of cases with a mild or early case of hyperthyroidism, have a T4 concentration remaining in a normal reference range. In addition, cats with concurrent nonthyroidal illness can have suppressed high serum T4 concentrations to within the normal reference range."

 

One such concurrent illness is CKD. Diagnosis of hyperthyroidism in cats with mild chronic kidney disease (2008) Wakeling J, Moore K, Elliott J & Syme H Journal of Small Animal Practice 49(6) pp287-94 found that the cats in the study with both CKD and signs consistent with hyperthyroidism nevertheless had normal T4 levels.

 

Be sure your vet allows for this when interpreting test results, especially if your cat already has CKD.

 

Cats in the grey zone may or may not need treatment, depending upon whether symptoms are present. 2016 AAFP guidelines for the management of feline hyperthyroidism (2016) Carney HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR Journal of Feline Medicine & Surgery 18(5) pp400-416 advise:

  • If T4 is normal but there are clinical signs of hyperthyroidism, you should repeat the T4 test and also perform a free fT4 test after 2–4 weeks. You should also consider T3 suppression or scintigraphy tests, and evaluate the cat for other diseases that may be causing some of the symptoms (e.g. CKD).

  • If T4 is normal and there are no clinical signs of hyperthyroidism, but the thyroid gland is enlarged, you should monitor the cat's symptoms and repeat the T4 test in six months.

T4: Low


A T4 level below normal, indicating hypothyroidism or an underactive thyroid, is extremely rare in cats. There are usually two main reasons for this:

  • Cats who are receiving treatment for hyperthyroidism may have been overtreated, so the opposite problem of an underactive thyroid occurs. This may happen after I-131 treatment in particular. Often this is mild and transient and will not need treatment, but some cats do need treatment, see below.

  • Cats with a low T4 level who also have another illness such as CKD may have Non-Thyroidal Illness Syndrome (though these cats tend to have T4 in the low normal range rather than in the below normal range).


Free T4 by Equilibrium Dialysis


 

Occasionally both T4 and T3 will appear normal, but you still suspect your cat may have hyperthyroidism. This happened to us with Ollie (who did not have any other diseases at the time): he had so many symptoms of hyperthyroidism but his T4  and T3 tests came back as completely normal.

 

In such cases you should ask your vet for a free T4 by equilibrium dialysis test. This test measures the amount of T4 available for use in the body. There is little point in running this test if your cat is firmly in the normal T4 range, or if your cat is in the likely hyperthyroidism range, but it can be helpful with borderline cases.

 

This test should not be relied upon in isolation (it should be run in conjunction with T4 tests), but it may assist with making a diagnosis. Confirming the diagnosis of hyperthyroidism: the T3 supporession test (2011) Peterson ME Insights into Veterinary Endocrinology says "In the majority of these hyperthyroid cats in which a normal total T4 concentration is found, simply repeating the total T4 analysis with simultaneous measurement of free T4, as well as ruling out any concurrent disease, will easily confirm the diagnosis."

 

Antech Diagnostics stated (2008) "The combined measurement of T4 over 2.3 ng/dL [over 30 nmol/L in international values] and FT4 over 40 mmol/L or with TSH under 0.03 ng/mL is of diagnostic value for the confirmation of hyperthyroidism in any cat with mild or previously diagnosed chronic kidney disease."

 

Around 70% of cats in the grey zone for T4 but with a high free T4 level will in fact have hyperthyroidism. 2016 AAFP guidelines for the management of feline hyperthyroidism (2016) Carney HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR Journal of Feline Medicine & Surgery 18(5) pp400-416 say "A T4 value in the upper half of the reference interval combined with an elevated fT4ed supports a diagnosis of hyperthyroidism."

 

Ollie's free T4 test came back indicating that he did in fact have hyperthyroidism. Once he was placed on medication, he improved greatly.

 

You may be wondering why the free T4 test is not run instead of the T4 test. What is T4? How is the T4 test used to diagnose hyperthyroidism in my cat? (2010) Peterson ME Insights into Veterinary Endocrinology says "the free T4 test is a more sensitive diagnostic test than is the T4 test, i.e., the free T4 test will be diagnostic in 98.5% of hyperthyroid cats whereas the total T4 test will be diagnostic in 91% of cats with the disease. However, many cats suffering from other illnesses NOT associated with hyperthyroidism will have false-positive results with the free T4 test. For that reason, the total T4 test remains the diagnostic test of choice for cats with suspected hyperthyroidism because we almost never see false-positive results with the total T4 test."

 

What if the free T4 level is normal? 2016 AAFP guidelines for the management of feline hyperthyroidism (2016) Carney HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR Journal of Feline Medicine & Surgery 18(5) pp400-416 say "A normal free T4 makes the diagnosis of hyperT unlikely, but if a strong clinical suspicion remains, repeat thyroid testing in 6 to 8 weeks or a technesium scan is indicated."

 

Focus on thyroid testing (2011) Nichols R Antech Insights Jan 2011 discusses the free T4 test.

 

Idexx Laboratories launched a new free T4 test in 2012 which is not performed by equilibrium dialysis. This means it can be turned around more quickly than the usual test and apparently it is just as reliable. I am awaiting more feedback on this test.

 


T3 Suppression Test


 

Some vets also run a T3 suppression test, which looks at levels of another hormone, T3 (tri-iodothyronine). Cats with hyperthyroidism usually have elevated T3 levels.

 

Frequently asked questions: thyroid function in cats Michigan State University Diagnostic Center for Population and Animal Health explains more about how the test is run. In a hyperthyroid cat, levels of both T3 and T4 are usually elevated, although sometimes only one of them is higher than normal.

 

This test seems to be falling out of favour. Confirming the diagnosis of hyperthyroidism: the T3 suppression test (2011) Peterson ME Insights into Veterinary Endocrinology discusses the T3 suppression test but points out its disadvantages include the test being lengthy (it is run over three days, with the caregiver having to pill the cat every eight hours for two days). 

 


TSH (Thyroid Stimulating Hormone)


 

Another test that may be used when the T4 results are not clearcut is the TSH test. This test is commonly used in humans to check for thyroid problems, but was not commonly used in cats because no assay had been developed for use in cats. Recently it appears that canine TSH tests may be applicable to cats, so some vets now run this test. This test cannot be used on its own, but may be of some use when used in addition to other tests.

 

Cats with hyperthyroidism would be expected to have very low levels of TSH, whereas healthy cats should have measurable levels. Evaluation of serum thyroid-stimulating hormone concentration as a diagnostic test for hyperthyroidism in cats (2015) Peterson ME, Guterl JN, Nichols R & Rishniw M Journal of Veterinary Internal Medicine 29(5) pp1327-34 measured TSH in both healthy cats and hyperthyroid cats. The study found that 69.9% of cats without thyroid problems had measurable levels of TSH, but so did 2% of the cats with hyperthyroidism. Conversely, 98% of hyperthyroid cats had undetectable levels of TSH, but so did 30% of older healthy cats. The study concludes "Measurement of serum TSH represents a highly sensitive but poorly specific test for diagnosis of hyperthyroidism and is best measured in combination with T4 and fT4."

 

That is to say, if a cat has a high T4 or free T4 level and undetectable levels of TSH, this would support the likelihood of hyperthyroidism, whereas if a cat has T4 or free T4 in the grey zone but has measurable levels of TSH, hyperthyroidism is less likely.

 

The TSH test is sometimes used in cats who have developed hypothyroidism following treatment for hyperthyroidism (see below).

 


Thyroid Scintigraphy (Technetium Scan)


 

Some centres offer scintigraphy to diagnose hyperthyroidism. This is often offered prior to I-131 treatment so the treatment can be tailored to the cat's needs, but may also be offered if there is some debate over whether your cat has hyperthyroidism or not.

 

During the scan, a radioactive solution (technetium) is injected into a vein and is taken up by the thyroid. Apart from confirming the presence of hyperthyroidism, this test also allows the precise areas of concern to be seen.

 

The scan only takes about 20-30 minutes, but because radioactive substances are used, the cat usually needs to be kept in the hospital for 24 hours after the scan.

 

The main downsides of thyroid scintigraphy are limited availability (it is usually only available at centres which perform I-131 treatment) and cost (it seems to cost at least US$500).

 

Thyroid scintigraphy Peterson ME explains more about scintigraphy.

 

Hyperthyroidism in cats University of California at Davis School of Veterinary Medicine explains more about thyroid scintigraphy.

 

Confirming the diagnosis of hyperthyroidism in cats: thyroid scintigraphy (2011) Peterson ME Insights into Veterinary Endocrinology explains the benefits of the procedure and how it is performed.

 

Thyroid scintigraphy findings in 2096 cats with hyperthyroidism (2016) Peterson ME & Broome MR Veterinary Radiology & Ultrasound 56(1) pp84–95 discusses the findings of thyroid scintigraphy.

 


Treatments Overview


 

There are a number of different treatments available, all with pros and cons. This section describes the available treatments, the risks of these treatments and the precautions which should be taken to reduce those risks.

Why Treatment is Essential


It is essential to treat hyperthyroidism because it is such a strain on the cat's body, particularly the heart. Effects of feline hyperthyroidism on kidney function: a review (2016) Vaske HH, Schermerhorn T & Grauer GF Journal of Feline Medicine & Surgery 18(2) pp55-9 states "Untreated, hyperthyroidism is eventually fatal."

 

2016 AAFP guidelines for the management of feline hyperthyroidism (2016) Carney HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR Journal of Feline Medicine & Surgery 18(5) pp400-416 say "Hyperthyroidism in cats is a life-threatening disease requiring prompt veterinary attention."

 

In addition, Feline hyperthyroidism and its relation with renal function (2006) Daminet S Presentation to the 31st World Small Animal Veterinary Association World Congress says "The presence of a hyperthyroid state could contribute to the development or progression of CRF."

 

Treatment Risks


Treating hyperthyroidism is usually safe and effective. The main concern is that doing so it may unmask previously hidden CKD (see above) or may worsen existing CKD (though what is actually happening is that the true degree of renal function is becoming apparent). This is a very real concern: Hyperthyroidism and the kidney (2006) Langston CE & Reine NJ Clinical Techniques in Small Animal Practice 21(1) pp17-21 says "overt renal failure occurs in approximately 30% of cats treated for hyperthyroidism".

 

Unfortunately it is difficult to predict which cats will go on to exhibit kidney problems after treatment for hyperthyroidism. An investigation of predictors of renal insufficiency following treatment of hyperthyroidism in cats (2008) Riensche MR, Graves TK, Schaeffer DJ Journal of Feline Medicine & Surgery 10(2) pp160-6 states that "our study suggests that the results of routine pre-treatment clinical data cannot be used to reliably predict renal function after treatment for hyperthyroidism."

 

A cat treated for hyperthyroidism may also develop the opposite problem of hypothyroidism, an underactive thyroid. Hypothyroidism can damage the kidneys further so it is essential to try to reduce the risk of this happening.

 

Because of these concerns, some vets may decline to treat hyperthyroidism in CKD cats, or may undertreat the condition. This is not recommended because of the risks of leaving hyperthyroidism untreated (see above). 2016 AAFP guidelines for the management of feline hyperthyroidism (2016) Carney HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR Journal of Feline Medicine & Surgery 18(5) pp400-416 state "The Panel recommends treatment of hyperthyroid patients regardless of concurrent disease – including cats with CKD."

 

In any event, declining to treat will not protect the kidneys. The guidelines go on to say: "Keeping cats with azotemia ‘a little bit’ hyperthyroid to increase renal perfusion and lower creatinine levels is deleterious. This approach can exacerbate renal damage while giving a false sense of security based on an artificially lowered creatinine level. Elevated T4 causes increased beta-adrenergic activity and activation of the renin–angiotensin–aldosterone system, leading to increased cardiac output, volume overload, sodium retention, renal hypertension and glomerular sclerosis, ultimately progressing to, or worsening, CKD."

 

It must also be remembered that hyperthyroidism is a progressive disease, so the longer you decline to treat it, the worse it will get and the greater the associated risks. Prevalence and degree of thyroid pathology in hyperthyroid cats increases with disease duration- a cross-sectional analysis of 2096 cats referred for radioiodine therapy (2016) Peterson ME, Broome MR & Rishniw M Journal of Feline Medicine & Surgery 18(2) pp92-103 found that "Median serum T4 concentration increased with increasing disease duration from 100 nmol/l (<1 year) to 315 nmol/l (>4-6.1 years." As you might expect, the tumours that cause hyperthyroidism also increased in size.

 

Therefore you should treat hyperthyroidism but it is wise to start with a conservative approach (see below).

 

Effects of methimazole on renal function in cats with hyperthyroidism (2000) Becker TJ, Graves TK, Kruger JM, Braselton WE and Nachreiner RF Journal of the American Animal Hospital Association 36 pp215-233 discusses the use of methimazole (a medication for hyperthyroidism in cats) and its effects on unmasking CKD.

 

Unmasking kidney disease in hyperthyroidism cats after treatment (2014) Peterson ME Insights into Veterinary Endocrinology discusses the unmasking of CKD.

 

The hyperthyroid cat with renal disease (proceedings) (2011) Gordon J DVM360 Magazine also discusses how CKD can be unmasked following treatment.

 

Treatment Goals


When treating feline hyperthyroidism, the target total T4 should be in the lower half of the reference interval. Generally speaking, the goal is a T4 in the low 2s ug/dl (USA) or around 25-30 umol/L (international). Some vets prefer to keep it a little higher than this in a CKD cat, but it should always be in the normal range. I would not let it fall below 1.5 μg/dl (20 umol/L) in a CKD cat.

 

How to dose and monitor hyperthyroid cats on methimazole (2012) Peterson ME Insights into Veterinary Endocrinology says "It is important to keep the serum T4 concentration within the mid-normal range and not have even mildly high or high-normal values on methimazole. For example, if the T4 reference range is listed as 0.8-4.0 μg/dl (10-50 nmol/L), my goal is maintain the T4 values between 1.5-2.5 μg/dl (20-32 nmol/L)."

 


Treatment Choices


 

All the available treatments are effective, even though some of them only control the hyperthyroidism rather than cure it. The costs are broadly similar too, assuming your cat lives for more than two years with hyperthyroidism.

 

The treatments all carry similar risks: Effects of feline hyperthyroidism on kidney function: a review (2016) Vaske HH, Schermerhorn T & Grauer GF Journal of Feline Medicine & Surgery 18(2) pp55-9 says "Regardless of the treatment modality used, when hyperthyroidism is successfully treated, there will be a decrease in renal excretory function, a decrease in the GFR, and an increase in serum creatinine."

 

Feline hyperthyroidism and its relation with renal function (2006) Daminet S Presentation to the 31st World Small Animal Veterinary Association World Congress has a table summarising the results of treating hyperthyroidism in a number of different studies in cats using the usual treatments.

 

2016 AAFP guidelines for the management of feline hyperthyroidism (2016) Carney HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR Journal of Feline Medicine & Surgery 18(5) pp400-416 outlines the pros and cons of the various treatments.

 

Drs Foster and Smith give examples of three cats who were each given a different treatment plan.

 


Non-Curative Treatments


 

These treatments will control the hyperthyroidism, and are often very effective, but they will not cure it.

 

The main advantage of these treatments is that you have room for manoeuvre if your cat develops CKD or if existing CKD worsens after treatment, because an adjustment of the dose is possible, which is not the case with the more permanent forms of treatment.


Hill's y/d Prescription Diet Food


 

Hill's introduced a new food called Hill's Prescription Diet y/d in 2011. It is available in both canned and dry formulas.

 

The food contains reduced levels of iodine. Iodine is necessary for the production of the thyroid hormones (T4 and T3), so the reasoning is that reduced levels of iodine mean reduced levels of thyroid hormones and thus the hyperthyroidism can be controlled.

 

Hill's claims that feeding the food exclusively for three weeks can control hyperthyroidism. Effect of feeding an iodine-restricted diet in cats with spontaneous hyperthyroidism (2015) Hui TY, Bruyette, DS, Moore GE & Scott-Moncrieff JC Journal of Veterinary Internal Medicine 29(4) pp1063-1068 looked into how effective this diet was. It found that 41.66% of the cats had normal thyroid values 21-60 days after beginning the diet, and after 61-180 days 83% of cats had thyroid values in the normal range. However, symptoms such as fast heart rate did not improve. The study concludes "Restricted-iodine diets were effective at maintaining serum TT4 concentrations within reference ranges for a majority of cats with spontaneous hyperthyroidism over 1 year, although not all clinical signs of hyperthyroidism improved."

 

Hill's state that this food should only be fed to the cat with hyperthyroidism, who can eat this food only and nothing else. The food does appear to do what it is supposed to, but will cease to have an effect if you do not feed it exclusively. Since hyperthyroidism is a progressive disease, it is possible that over time the food alone will not be sufficient to control your cat's hyperthyroidism.

 

Healthy cats with normal thyroid function should not be allowed to eat the food because it could cause problems.

 

Hill's y/d contains the following levels of phosphorus and protein,, which are within the usual guidelines for CKD cats:

 

Variety Phosphorus % (Dry Matter Analysis) Protein % (Dry Matter Analysis)
Hill's y/d canned 0.57 33.70
Hill's y/d dry 0.70 36.30

 

Why/d? Peterson ME Animal Endocrine Clinic gives Dr Peterson's opinion of Hill's y/d.

 


Medications: Methimazole and Carbimazole


 

Medications may be used used to regulate the production of thyroid hormone. The two main treatments are methimazole or carbimazole. In the USA carbimazole is not currently available so you will be offered methimazole. Antithyroid drug treatment for hyperthyroidism: brand name, generic or compounded drug? (2012) Peterson ME Insights into Veterinary Endocrinology gives an overview of the various drugs used for hyperthyroidism in cats and the forms in which they are available.

 

Medications are a relatively simple way of treating hyperthyroidism and are very effective but they can have side effects, and they only control the condition, they will not cure it. Therefore if you stop the treatment, your cat's hyperthyroidism will return. In addition, since hyperthyroidism is a progressive disease, you will probably find that you will need to increase the dose over time.

 

When handling any form of thyroid medication, you should wear gloves in order to avoid absorbing any of the treatment yourself. Veterinary Information Network has some information on this.  

 


Methimazole


 

Methimazole is the most commonly used thyroid medication in the USA, and is becoming more common in the UK.

 

Methimazole Formulations USA


Methimazole is available in pill, oral suspension and transdermal (ointment) form in the USA. The trade name for the feline pill version of methimazole is Felimazole but you might possibly be offered the human version, known as Tapazole.

 

Methimazole Formulations UK


Felimazole in pill form is also available in the UK. In 2017 a new liquid form was introduced called Thyronorm. It is available in 30ml and 100ml bottles with an in-use shelf life of six months. Thyronorm is flavoured "with added honey for even greater pet acceptance", which mystifies me because I don't know any cat who likes honey (cats lack the taste receptors for sweet). 

 

In the USA you also have the option of transdermal methimazole from compounding pharmacies. Transdermal medications are usually applied to the ears. Efficacy and safety of transdermal methimazole in the treatment of cats with hyperthyroidism (2004) Sartor LL, Trepanier LA, Kroll MM, Rodan I, Challoner L J Journal of Veterinary Internal Medicine 18(5) pp651-5 found that transdermal methimazole took longer to work than the oral version but appeared to have fewer side effects, so this might be one option if your cat does not tolerate oral methimazole very well.

 

Drs Foster & Smith have a video about giving the medication transdermally.

 

Hyperthyroid cats and transdermal methimazole is a helpful video which shows how one person administers transdermal medications to his cat.

 

Methimazole Dosing in Non-CKD Cats


I sometimes hear from people whose vets have them begin with a dose of 5 mg once or even twice a day. This may become more common in the UK because the recommended starting dose for Thyronorm is "5mg active ingredient per day, ideally split into two doses." This size of starting dose makes me nervous, and some cats on this dosing regimen can become quite ill, particularly if they had CKD before they began treatment for hyperthyroidism (though non-CKD cats may also be affected). Whilst these dosages are not excessive per se, it is much safer to start at a lower dose and build up gradually to the dose your cat requires, so the cat's body has time to adapt.

 

Diagnostic and treatment options for feline hyperthyroidism (2003), Nelson R Presentation to the World Small Animal Veterinary Association World Congress recommends starting at a dose of 2.5mg once a day for the first two weeks, increasing to 2.5mg twice a day if the cat is showing no adverse reaction and tests indicate that a higher dose is required (these doses are per cat, not per pound or kg).

 

Later guidelines, 2016 AAFP guidelines for the management of feline hyperthyroidism (2016) Carney HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR Journal of Feline Medicine & Surgery 18(5) pp400-416, suggest a lower dose may also be suitable, stating "Methimazole should be started at a dose of 1.25–2.5 mg per cat twice daily (q12h). Twice daily dosing is associated with less serious side effects than a higher dose once daily (q24h). After the cat becomes euthyroid with q12h dosing, giving the total daily dose q24h may maintain euthyroidism and increase owner compliance. Transdermal methimazole preparations, when available, can be useful for uncooperative cats. In such cases, the same or a slightly higher starting dose than for the oral route should be used."

 

The guidelines go on to say "Most hyperthyroid cats are euthyroid within 2–3 weeks of commencing treatment. If the cat is still hyperthyroid, methimazole dose adjustments can be made in increments of 1.25–2.5mg/day until euthyroidism is achieved."

 

If the cat's T4 falls too low, the guidelines advise "If T4 drops below the lower end of the reference interval, the methimazole dosage should be reduced in decrements of 1.25–2.5 mg/day and the T4 and renal parameters rechecked in 1 week."

 

How to dose and monitor hyperthyroid cats on methimazole (2012) Peterson ME Insights into Veterinary Endocrinology explains more about dosing and monitoring cats on methimazole. He says "It is important to keep the serum T4 concentration within the mid-normal range and not have even mildly high or high-normal values on methimazole. For example, if the T4 reference range is listed as 0.8-4.0 ug/dl (10-50 nmol/L), my goal is maintain the T4 values between 1.5-2.5 ug/dl (20-32 nmol/L)."

 

Methimazole Dosing in CKD Cats


For cats with pre-existing CKD, a lower starting dose is wise. How to treat a cat with hyperthyroidism and concurrent renal insufficiency (2010) Daminet S Presentation to the 35th World Small Animal Veterinary Association World Congress states "methimazole or carbimazole are used (orally or transdermal), at a low starting dose (i.e. 1.25 mg orally once a day). This allows assessing the impact of anti-thyroid therapy on renal function. These patients should be monitored every 2 weeks. Dosage adjustments should be made prudently".

 

Methimazole Side Effects and Overdosing


Side effects include lack of appetite, vomiting and in rare cases anaemia. Cats on methimazole may also have elevated liver enzymes and low white blood cells.

 

Veterinary Partner discusses possible side effects, including itching, particularly on the face.

 

Fortunately side effects usually do resolve. 2016 AAFP guidelines for the management of feline hyperthyroidism (2016) Carney HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR Journal of Feline Medicine & Surgery 18(5) pp400-416 state "Most side effects appear within the first 4–6 weeks of therapy and are less common after 2 or 3 months of treatment."

 

If your cat is on methimazole and you also wish to use mirtazapine (an appetite stimulant), I would talk to your vet about using a reduced dose of mirtazapine. This is because I have heard from a couple of people whose cats were on treatment for hyperthyroidism who did not seem to do well on mirtazapine. This is probably because methimazole reduces levels of a liver enzyme called CYP2D6 which helps to clear mirtazapine from the body. The University of Maryland Medical Center (search for both drugs) has some information about this.

 

Unfortunately, as Drs Foster & Smith says, "Signs of toxicity or overdose are similar to the side effects listed; liver and blood disorders are the most common."

 


Carbimazole


 

Carbimazole is another form of thyroid medication, which is commonly prescribed in the UK but only rarely used in the USA.

 

Carbimazole is actually converted into methimazole in the cat's body, but it tends to have fewer side effects, probably because carbimazole appears to act more slowly than methimazole and is absorbed less quickly. I have heard of quite a few cats who did badly on methimazole but who could tolerate carbimazole just fine, so if this applies to you, you may wish to try carbimazole instead and see if your cat does better on it.

 

Newman Veterinary mentions that there are fewer side effects with carbimazole than with methimazole.

 

Feline hyperthyroidism: spectrum of clinical presentations and response to carbimazole therapy (2000) Bucknell DG Australian Veterinary Journal 78(7) mentions that there is less chance of side effects with carbimazole than with methimazole.

 

Carbimazole Formulations USA


Carbimazole can be obtained in the USA from compounding pharmacies (see Tips on Medicating Your Cat).

 

Carbimazole Formulations UK


Generic carbimazole used to be the drug treatment of choice for hyperthyroid cats in the UK, but once veterinary formulations of thyroid medications became available (which you will not be surprised to hear are much more expensive than generic carbimazole), vets became legally obliged to prescribe these first. This is because UK regulations require that a medication approved for a species in the UK must be used as the first choice, and alternatives can only be considered if the first choice medication does not work or causes too many side effects. This policy is known as the cascade and The British Small Animal Veterinary Association explains more about how it works.

 

Therefore you will most probably be offered either the veterinary version of carbimazole called Vidalta, or methimazole with the brand name of Felimazole or Thyronorm. Personally I would always opt to use carbimazole over methimazole.

 

Vidalta is only available in 10mg or 15mg tablets which are slow release, so they should not be crushed. Some people were concerned about how to use Vidalta in cats because of this, bearing in mind that many cats need a relatively small dose. However, in How to dose and monitor hyperthyroid cats on methimazole (2012) Peterson ME Insights into Veterinary Endocrinology mentions that "it is estimated that 15 mg of this preparation is equivalent to approximately 7.5 mg of conventional methimazole", but that absorption is higher if given with food.

 

If you have any problems with the branded products, your vet can still get the generic carbimazole for you, and it is available in a 5mg size. I used generic carbimazole for my Ollie simply because he tolerated it better (as a bonus, it saved me money). See below for dosing.

 

Carbimazole Dosing


Dosing is similar to that for  methimazole, although carbimazole may be given up to three times a day if necessary (i.e. the total daily dose can be divided into three).

 

Having said that, Pharmacokinetics of controlled-release carbimazole tablets support once daily dosing in cats (2008) Frénais R, Burgaud S, Horspool LJ Journal of Veterinary Pharmacology and Therapeutics 31(3) pp213-9, found that Vidalta was effective when given only once a day. A further study, Clinical efficacy and safety of a once-daily formulation of carbimazole in cats with hyperthyroidism (2009) Frénais R, Rosenberg D, Burgaud S, Horspool LJ Journal of Small Animal Practice 50(10) pp510-5 also concluded that once daily dosing was effective. These studies were conducted by the manufacturer of Vidalta.

 


Curative Treatments


 

These treatments will cure the hyperthyroidism, and are usually very effective.

 

The main disadvantage of these treatments is that you have less room for manoeuvre if your cat develops CKD or if existing CKD worsens after treatment, because an adjustment of the dose is not possible as it is with medication or diet.

Medication Trial


Some vets recommend a medication trial before opting for curative treatment. This means that the hyperthyroidism is treated initially with medication (methimazole or carbimazole). This enables you to adjust the dose should you see any signs of problems in an attempt to balance the conflicting needs of the kidneys and the thyroid, and ideally avoid a crash; whereas this is not possible with the curative treatments, where the treatment is complete and no finetuning is possible.

 

Feline hyperthyroidism 2009 (2009) Graves TK CVC in Washington DC Proceedings explains more about using medication as the first choice treatment and recommends doing this for at least thirty days.

 

Feline hyperthyroidism and its relation with renal function (2006) Daminet S Presentation to the 31st World Small Animal Veterinary Association World Congress says "It seems reasonable (although not evidence-based) to recommend a trial therapy in any hyperthyroid cat presented with one or more of the following: BUN or serum creatinine values at the high end of the reference interval, a low USG, an increased UPC or marked ultrasonographic kidney abnormalities. Currently the most useful predictive parameter seems to be GFR measurement. A low pre-treatment GFR was predictive of the development of CRF in several studies. However, measurement of GFR is often impractical in a clinical setting."

 

2016 AAFP guidelines for the management of feline hyperthyroidism (2016) Carney HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR Journal of Feline Medicine & Surgery 18(5) pp400-416 differentiate between cats depending upon their stage of CKD, saying "Manage IRIS stage 1 and 2 cases as though they are non-azotemic. If the patient responds favorably and renal function is stable using a reversible treatment, then consider an irreversible FHT treatment. IRIS stage 3 and 4 patients warrant a more prudent approach consistent with Group 5 status; for example, using lower doses of methimazole and more aggressive management of CKD. If a permanent treatment for FHT is pursued, careful monitoring and aggressive kidney support may be required during the period of regeneration of previously suppressed normal thyroid tissue."

 

Such a trial may also be wise in cats with some other conditions. 2016 AAFP guidelines for the management of feline hyperthyroidism (2016) Carney HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR Journal of Feline Medicine & Surgery 18(5) pp400-416 say "Hyperthyroid cats at increased risk of complications, including those with cardiovascular disease or severe hyperthyroidism, may benefit from treatment with methimazole or beta-adrenergic antagonists before definitive treatment with radioactive iodine or surgery."

 

Personally, I would want to do a trial in any of my cats.

 


Surgery (Thyroidectomy)


 

Surgical removal of the thyroid gland, or thyroidectomy, is a possible treatment option for hyperthyroidism. Although this is usually successful, it is of course invasive, exposing the cat to the risks of anaesthesia. 2016 AAFP guidelines for the management of feline hyperthyroidism (2016) Carney HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR Journal of Feline Medicine & Surgery 18(5) pp400-416 state "Surgery and anesthesia are sometimes associated with substantial procedural morbidity and mortality. Hypocalcemia occurs in a widely varying range (6–82%) of thyroidectomy patients, depending on the surgical method chosen."

 

There is also a risk that too much of the gland may be removed, resulting in the opposite problem, an underactive thyroid (hypothyroidism). Complications of thyroidectomy in cats: post-operative hypothyroidism (2012) Peterson ME Insights into Veterinary Endocrinology says "Most hyperthyroid cats are readily cured quite easily with the use of surgical thyroidectomy" but goes on to say that most of them will develop hypothyroidism, which may be temporary or permanent.

 

2016 AAFP guidelines for the management of feline hyperthyroidism (2016) Carney HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR Journal of Feline Medicine & Surgery 18(5) pp400-416 nevertheless state "Surgical thyroidectomy is associated with a high rate of both short- and long-term success, with most studies showing >90% of cats achieving euthyroidism postoperatively, with a relapse rate approaching 5% within 3 years."

 

I hear from very few people whose cats undergo this treatment these days, and it would not be my first choice.

 


Radioactive Iodine Treatment (I-131)


 

Radioactive iodine treatment (I-131) is considered by some to be the "gold standard" treatment for hyperthyroidism. 2016 AAFP guidelines for the management of feline hyperthyroidism (2016) Carney HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR Journal of Feline Medicine & Surgery 18(5) pp400-416 say "Experts generally agree that radioiodine is the treatment of choice for most cats with FHT."

 

The treatment entails the cat being given a dose of radioactive iodine, which destroys the diseased thyroid tissue while leaving healthy thyroid tissue intact. The treatment is very effective: Risks associated with radioiodine therapy for feline hyperthyroidism Advanced Veterinary Medical Imaging states 'The overwhelming majority of cats (>95%) are cured by a single therapy with only a small percentage of cats requiring additional radioiodine therapy (4%) or becoming persistently hypothyroid (1%)."

 

The treatment has no major side effects, but it is only available in a limited number of facilities, and the cat has to be hospitalised for a period ranging from a couple of days to a week or more in some locations. 

 

The main downside with this treatment is that many cats develop hypothyroidism (an underactive thyroid) after treatment, which may damage the kidneys. Fortunately in most cases the hypothyroidism is temporary, but some vets who are prepared to give I-131 treatment to cats who already have CKD believe that cats with CKD should be treated with L-thyroxine (a treatment for hypothyroidism) during this period of stabilisation, so as to prevent further damage to the kidneys. Feline hyperthyroidism - avoiding further renal injury Broome MR discusses this.

 

Radioactive Iodine Treatment (I-131) Dosage


Cats used to be given a "standard" dose of 4-5 millicuries of radioactive iodine which was too much for some cats and which made them hypothyroid. It is now thought better to tailor the dose to the individual cat.

 

Efficacy of low-dose (2 millicurie) versus standard-dose (4 millicurie) radioiodine treatment for cats with mild-to-moderate hyperthyroidism (2017) Lucy JM, Peterson ME, Randolph JF, Scrivani PV, Rishniw M, Davignon DL, Thompson MS & Scarlett JM Journal of Veterinary Internal Medicine 31(2) pp326-34 compared a lower dose to the standard dose. It found that "There was no significant difference in prevalence of cats with persistent hyperthyroidism between standard- and low-dose treatment groups at 3 and 6 months. Overt or subclinical hypothyroidism was more common in cats at 6 months after standard-dose 131I. No difference in incidence of azotemia existed between groups, but cats treated with standard-dose 131I had higher creatinine concentrations and higher percent rises in creatinine." The study concludes that the lower dose is safe and effective, with a cure rate of over 95%.

 

Radioactive Iodine Treatment (I-131) Requirements


The cat needs to stop any medication for hyperthyroidism two weeks before the I-131 treatment.

 

Because the treatment is radioactive, the cat needs to remain hospitalised until s/he is at a safe level. The time period is determined by the authorities where the treatment is performed, not by the treating facility, and varies from country to country and state to state. It can be as long as a week in some countries such as Australia, but is just a couple of days in some locations. During this period you cannot touch your cat, so you could consider leaving an item of clothing with your smell on to comfort your cat (take an old item because it cannot be returned).

 

Following the treatment, the cat must be kept indoors for two weeks and away from pregnant women and children.

 

Flushable litter must be used or the litter must be stored and disposed of in accordance with local laws. Alternate litter box management options following radioiodine therapy for feline hyperthyroidism Advanced Veterinary Medical Imaging explains more about this.

 


Post-Treatment: Monitoring


 

It is essential to monitor cats after treatment for hyperthyroidism because so many of them (15-49%, according to Effects of feline hyperthyroidism on kidney function: a review (2016) Vaske HH, Schermerhorn T & Grauer GF Journal of Feline Medicine & Surgery 18(2) pp55-9), will have worsening of their kidney function.

 

Short- and long-term follow-up of glomerular and tubular renal markers of kidney function in hyperthyroid cats after treatment with radioiodine (2009) van Hoeka I, Lefebvreb HP, Peremansa K, Meyera E, Croubelsa S, Vandermeulena E, Kooistrac H, Saunders JH, Binsta D & Damineta S Domestic Animal Endocrinology 36(1) pp45-56 found that "significant changes in kidney function occur within 4 weeks post-treatment and none thereafter."

 

However, Effects of feline hyperthyroidism on kidney function: a review (2016) Vaske HH, Schermerhorn T & Grauer GF Journal of Feline Medicine & Surgery 18(2) pp55-9) states that creatinine may "continue to increase for 6 months after the cat becomes euthyroid, while GFR can decrease for up to one month following attainment of euthyroidism, and then will stabilize."

 

Your cat should be monitored for at least six months once s/he is euthyroid. Since T4 is often lowered in cats with non-thyroidal illness syndrome, TSH should also be assessed.

 

What's the expected time for signs of feline hyperthyroidism to resolve after treatment? (2014) Peterson ME Insights Into Veterinary Endocrinology states "To monitor for iatrogenic hypothyroidism, we routinely run a serum thyroid panel (i.e., total T4, T3, free T4, and TSH) at 1 and 3 months after treatment. Most facilities recommend monitoring just the total T4 concentration, but this is not adequate for monitoring since many hypothyroid cats will maintain a low-normal total T4 value, despite being hypothyroid.  Based on our studies, it's becoming increasing clear that feline hypothyroidism can only be diagnosed by finding low to low-normal T4 and T3 values in conjunction with high TSH values."

 

Hypothyroidism


It is particularly important to monitor for hypothyroidism (an underactive thyroid) in cats treated for hyperthyroidism because hypothyroidism can damage the kidneys. Transient hypothyroidism in cats following radioiodine (2013) Peterson ME Insights Into Veterinary Endocrinology explains more about why this happens.

 

Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism (2010) Williams TL, Elliott J & Syme HM Journal of Veterinary Internal Medicine 24(5) pp1086-92 found that "iatrogenic hypothyroidism appears to contribute to the development of azotemia after treatment of hyperthyroidism, and reduced survival time in azotemic cats." Iatrogenic means the hypothyroidism was caused by a curative treatment for hyperthyroidism, i.e. surgery or I-131 treatment.

 

Although up to 75% of cats treated with I-131 will become hypothyroid, fortunately in most cases this is not permanent. 2016 AAFP guidelines for the management of feline hyperthyroidism (2016) Carney HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A & Rucinsky AR Journal of Feline Medicine & Surgery 18(5) pp400-416 state "up to 75% of cats may become hypothyroid for some interval post-therapy. Because 131I predominantly damages hyperactive cells, permanent post-treatment hypothyroidism is an uncommon sequela."

 

Risks associated with radioiodine therapy for feline hyperthyroidism Advanced Veterinary Medical Imaging states "with only a small percentage of cats...becoming persistently hypothyroid (1%)."

 

However, some cats may need to be treated for hypothyroidism while it is present, particularly CKD cats, because hypothyroidism damages the kidneys. In some cases the cat may need to be on treatment for hypothyroidism on a permanent basis (in the form of a daily pill of levothyroxine). When to start thyroid hormone replacement in cats treated with radioiodine (I-131) (2015) Peterson ME Insights Into Veterinary Endocrinology explains when he treats for hypothyroidism.

 


Prognosis


 

Long term health and predictors of survival for hyperthyroid cats treated with iodine 131 (2001) Slater MR, Geller S, Rogers K Journal of Veterinary Internal Medicine 15 pp47-51 is a study of the survival rate of 231 cats treated with the I-131 treatment which found that having kidney disease adversely affected the chances of survival following this type of treatment.   

 

This is in part because of the risks of I-131 treatment causing  the opposite problem of hypothyroidism. Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism (2010) Williams TL, Elliott J & Syme HM Journal of Veterinary Internal Medicine 24(5) pp1086-92 found that "Iatrogenic hypothyroidism appears to contribute to the development of azotemia after treatment of hyperthyroidism, and reduced survival time in azotemic cats."  (Iatrogenic hypothyroidism means hypothyroidism caused by I-131 or surgery).

 

However, overall hyperthyroidism treatments are safe and effective in most cases. Survival times for cats with hyperthyroidism treated with iodine 131, methomazole, or both; 167 cases (1996-2003) (2006) Milner RJ1, Channell CD, Levy JK & Schaer M Journal of the American Veterinary Medical Association 228(4) pp559-63 assessed various treatment options. 14% of the cats in the study had CKD before treatment. The study states "Age was positively correlated (r = 0.4) with survival time, with older cats more likely to live longer. Cats with preexisting renal disease had significantly shorter survival times than did cats without preexisting renal disease. When cats with preexisting renal disease were excluded, median survival time for cats treated with methimazole alone (2.0 years; interquartile range [IQR], 1 to 3.9 years) was significantly shorter than median survival time for cats treated with 131I alone (4.0 years; IQR, 3.0 to 4.8 years) or methimazole followed by 131I (5.3 years; IQR, 2.2 to 6.5 years)."

 

Effects of feline hyperthyroidism on kidney function: a review (2016) Vaske HH, Schermerhorn T & Grauer GF Journal of Feline Medicine & Surgery 18(2) pp55-9 states "The reported median survival time for cats with hyperthyroidism ranges from 1.6–4.0 years, while cats classified as having CKD prior to treatment of hyperthyroidism have shorter survival times of 0.5–2.0 years."

 

Survival of hyperthyroid cats is not affected by post-treatment azotemia (2006) Wakeling J, Rob C, Elliott J, Syme H Journal of Veterinary Internal Medicine 20 p1523 states that the "mild azotemia that can occur after treatment of hyperthyroidism does not affect outcome... It would appear that there is no indication to discontinue therapy for hyperthyroidism if mild azotemia develops with therapy. The survival time after therapy is relatively long in most cats considering that hyperthyroid cats represent geriatric pets."

 

Feline hyperthyroidism - avoiding further renal injury Broome MR states "Most cats with concurrent CKD and hyperthyroidism will benefit from the administration of subcutaneous fluids following the initiation of antithyroid therapy."

 

If I had a cat with hyperthyroidism, I would treat him or her.

 


Holistic Treatments


 

I know some people like to use holistic treatments so I am mentioning a few here. I do not know anybody who has used them, and it must be accepted that although some may possibly seem to work for a short while, since hyperthyroidism is progressive, eventually you will need to use an allopathic treatment; and while you've been delaying that, your cat's kidneys will be deteriorating and you may be putting your cat at risk of heart problems and strokes.

 

If you wish to take the homeopathic route, you must use a qualified vet, not just buy the remedy over the counter and try to treat yourself.

 

Homeopathic treatment of feline hyperthyroidism (2006) Dobias P Presentation to the 31st World Small Animal Veterinary Association World Congress reports on the use of a homeopathic remedy, Natrum Muriaticum, to treat hyperthyroidism in 13 cats. Results were mixed, but it did appear to be effective for eight (61.5%) of the cats. It appeared to be most effective when used in relatively young cats (average age of the group who responded positively to the treatment was 7.4) who were diagnosed early.

 

L-carnitine may be of some limited use in controlling the symptoms of hyperthyroidism, at least initially, but like other holistic treatments it will not reduce your cat's T4 levels or the size of the tumour causing the hyperthyroidism.

 

Alternative medical treatments for hyperthyroid cats (2012) Peterson ME Insights Into Veterinary Endocrinology says "Personally, I have never seen a cat treated with one of these natural alternative medications that was cured or has clinical signs resolve, at least on a long-term basis. Most of the active ingredients in the herbs act by supposedly lowering TSH secretion, but circulating TSH is already low in these cats with hyperthyroidism. It is also important to remember that cats with hyperthyroidism usually have one or more thyroid adenomas and, less commonly, thyroid carcinoma. No one has ever documented  shrinkage of the thyroid tumor or significant lowering of the serum T4 value after these alternative medications."

 


Diet


 

Diet and nutritional management for hyperthyroid cats (2011) Peterson ME Insights into Veterinary Endocrinology states that hyperthyroid cats usually benefit from a diet composed of around 50% protein. He writes "This absolute requirement for dietary protein intake in cats is critically important when formulating a diet for hyperthyroid cats, in which protein catabolism and muscle wasting is universally present."

 

The best diet to feed hyperthyroid cats (2011) Peterson ME Insights into Veterinary Endocrinology discusses weight and muscle loss in cats and says, "Like normal and diabetic cats, I believe that it makes sense to feed most hyperthyroid cats a diet composition close to what they would be getting in the wild. That would be a diet composed of approximately 50-60% protein, 5-10% carbohydrates, and 30-50% fat."

 

For cats with both CKD and hyperthyroidism, Dr Peterson states in Dietary management of cats with kidney disease after treatment with radioiodine for hyperthyroidism (2013) that reducing protein should be avoided for cats in IRIS stages 1 and 2, but that phosphorus control is very important. This is in line with the recommendation for cats with CKD only.

 


Related Health Concerns


 

Hyperthyroidism may cause a number of different problems which may need additional treatment, as follows:

 

Hypertension


Many cats with hyperthyroidism also have hypertension, which may improve once the hyperthyroidism is under control. If your cat continues to have hypertension despite getting the hyperthyroidism under control, you should discuss the need for medication with your vet. Untreated hypertension is very dangerous.

 

Heart Problems


If your cat also has heart problems that do not resolve following treatment for hyperthyroidism, please read the Heart Problems page and discuss how best to manage them with your vet.

 

Urinary Tract Infections


Hyperthyroidism: a view from the urinary tract (2010) Grauer GF CVC in Kansas City Proceedings says "urinary tract infection (UTI) can be consequences of either hyperthyroidism or CKD..."Urine cultures should be obtained as part of the workup of both hyperthyroidism and CKD. In either case, a concurrent UTI should be managed as a complicated UTI with long-term antibiotic treatment based on culture and sensitivity results."

 

Vitamin B12 (Cobalamin)


The prevalence of hypocabalaminaemia in cats with spontaneous hyperthyroidism (2011) Cook AK, Suchodolski JS, Steiner JM & Robertson JE Journal of Small Animal Practice 62(2) pp101-6, found that hyperthyroid cats may have low cobalamin (Vitamin B12) levels. I would therefore discuss Vitamin B12 supplementation with your vet.


Support


 

Feline HyperT Group is a support group for people with hyperthyroid cats, where you can obtain feedback on treatments, and support on living with hyperthyroidism.

 

Feline Thyroid Management Group is a smaller group which tends to focus more on cats who have developed hypothyroidism following treatment for hyperthyroidism.

 

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This page last updated: 28 August 2017

Links on this page last checked: 28 August 2017