Low Phosphorus Levels (Hypophosphataemia)

Low phosphorus levels (below 3 mg/dl or 1.0 mmol/L) are extremely uncommon in CKD cats, though are occasionally seen in diabetic cats on insulin, or in cats with certain kinds of cancer. They may also be seen in a cat who is being fed following a prolonged period of starvation.

If the phosphorus level is too low as a result of using phosphorus binders, you can reduce the dose of binder that you are using. Talk to your vet about the best approach.

Low Calcium Levels (Hypocalcaemia)

High Calcium Levels: Simple Treatments

These treatments are simple in that you will probably be using them to help with other problems anyway, or they do not require much effort to implement. You may well find that you need to use more than one treatment.

• Metabolic acidosis

• Subcutaneous fluids

• Phosphorus binders

• Limiting products with calcium or Vitamin D

• Dietary changes

• Fibre, including chia seeds

See below for more advanced treatments.'

High Calcium Levels: Metabolic Acidosis

Metabolic acidosis (disturbance of the acid-base in the body) can contribute to hypercalcaemia, so if your cat has metabolic acidosis, treating it can help reduce calcium levels.

High Calcium Levels: Subcutaneous Fluids (Sub-Qs)

Keeping your cat properly hydrated is one of the main goals of CKD treatment, so most cats with creatinine over 3.5 mg/dl  or 300 µmol/L will be receiving sub-Qs for this reason. As it happens, giving sub-Qs may also reduce calcium levels, though it is not usually recommended to use sub-Qs solely in order to control calcium levels.

Pitfalls in the management of idiopathic hypercalcemia in cats (2013) Peterson ME Proceedings of the 2013 American College of Veterinary Internal Medicine (ACVIM) Forum pp538-540 says "Fluid therapy is used as an adjunct treatment in cats with idiopathic hypercalcemia, especially those that have developed renal failure. The administration of subcutaneous fluids on a daily or every other day basis may help expand the extracellular fluid, and promote calciuresis.”

Normally cats on sub-Qs with high calcium levels are given saline solution rather than lactated ringers solution (LRS). This is for two reasons: firstly, saline solution has no added calcium, and secondly, saline solution promotes calcium excretion in the urine. Alternatively, Normosol-R might be suitable. If you are using LRS, discuss switching with your vet.

High Calcium Levels: Phosphorus Binders

Since phosphorus and calcium levels within the body are closely related, reducing elevated phosphorus levels may also help reduce calcium levels.

Even if your cat's calcium levels are normal, you need to take steps to control your cat's phosphorus level if it is over 6 mg/dl or 1.9 mmol/L, see The Importance of Phosphorus Control.

Never use calcium-based phosphorus binders in a cat with elevated calcium levels.

High Calcium Levels: Limit Treatments Containing Calcium or Vitamin D

Slippery elm bark contains calcium, so it is probably safer not to use it if your cat has hypercalcaemia.

Some people have found that using lactulose to control constipation has led to hypercalcaemia in their cats. This may be coincidence, since hypercalcaemia is not uncommon in CKD cats, but you may wish to avoid lactulose if your cat already has hypercalcaemia, and consider alternative treatments if your cat has developed hypercalcaemia since beginning lactulose. See All About Constipation for more information and alternative treatments for constipation.

Also check for any supplements containing vitamin D, and ensure that you are not feeding a food high in vitamin D.

High Calcium Levels: Dietary Changes

It is worth looking at the food you are feeding because some contain more calcium than others.

The body needs a balance between phosphorus and calcium, so foods usually have a particular ratio of calcium to phosphorus. The minimum is 1:1, though for healthy cats a higher ratio of calcium to phosphorus is not normally a problem. Standard commercial diets tend to have a ratio of around 1.2:1 (calcium to phosphorus).

Therapeutic kidney diets tend to contain more calcium in ratio terms because they are usually focused on keeping phosphorus levels low:

• Hill's k/d has a level of 1.7:1 canned and 1.43 dry;

• Purina NF is 1.68:1 for the dry and 1.36:1 for the canned;

• Royal Canin has much higher levels, at around 2:1;

but the overall amount of calcium is still usually low.

Hypercalcemia in chronic kidney disease (2024) van den Broek H International Renal Interest Society states "A diet change is the first step advised for management of cats with mild hypercalcemia. When persistent hypercalcemia occurs or is worsening in a cat fed a renal diet, and no other underlying cause has been identified, it is advisable to discontinue feeding 100% renal diet. Either the proportion of renal diet fed on a daily basis could be reduced, or renal diet could be stopped completely. A dietary transition could then be made to a diet less stringently phosphate restricted, such as a senior diet, or to a calcium oxalate prevention diet, which is restricted in calcium."

If your cat is hypercalcaemic and in IRIS stages 1 and 2, discuss with your vet whether to feed a normal commercial diet with a lower calcium content rather than a therapeutic kidney diet. Cats in these stages do not normally need a reduced protein diet (see Nutritional Requirements), and in fact low protein and low phosphorus diets may not be the best choice, according to Clinical progression of cats with early-stage chronic kidney disease fed diets with varying protein and phosphorus contents and calcium to phosphorus ratios (2021) Schauf S, Coltherd JC, Atwal J, Gilham M, Carvell-Miller LJ, Renfrew H, Elliott J, Elliott D, Bijsmans ES, Biourge VC, Watson P & Bakke AM Journal of Veterinary Internal Medicine 35(6) pp2797-2811. In this study,  cats in IRIS stages 1 or 2 were fed a low protein, low phosphorus diet, followed by a moderate protein, moderate phosphorus diet. The study concludes "Cats with early CKD developed hypercalcemia after long-term feeding of a highly P-restricted diet. Increasing dietary P and reducing Ca : P ratio maintained renal markers, while improving Ca-P balance. Cats with early CKD could benefit from moderately protein- and P-restricted diets."  

However, it must not be forgotten that a therapeutic kidney diet has many features other than reduced protein levels (see Which Foods). Your vet can help you decide what to focus on.

If your cat does require a therapeutic kidney diet, I would check the calcium:phosphorus ratio carefully and aim for a food with a lower calcium:phosphorus ratio, assuming your cat will eat it. You might wish to add a food to it which is relatively low in calcium, in order to adjust the calcium:phosphorus ratio. You should also avoid feeding acidified diets.

Nutritional management of idiopathic hypercalcaemia in cats (2012) Peterson ME Insights into Veterinary Endocrinology discusses ways to treat idiopathic hypercalcaemia (idiopathic means no obvious cause can be found) through diet.

In Pitfalls in the management of idiopathic hypercalcemia in cats (2013) Peterson ME Proceedings of the 2013 American College of Veterinary Internal Medicine (ACVIM) Forum pp538-540, Dr Peterson also says "Unfortunately, in most cats with idiopathic hypercalcemia, dietary therapy will be of minimal long-term benefit, but this management is very unlikely to be harmful. In addition, because many cats with idiopathic hypercalcemia do not show clinical signs for months to years, it is appropriate to try dietary therapy as the first line of therapy at least in cats with mild ionized hypercalcemia."

High Calcium Levels: Fibre

Increased fibre in the diet may help to reduce calcium levels by binding with the calcium and thus reducing the amount of calcium that can be absorbed into the body through the gastrointestinal tract.

ACVIM small animal consensus recommendations on the treatment and prevention of uroliths in dogs and cats (2016) Lulich JP, Berent AC, Adams LG, Westropp JL, Bartges JW & Osborne CA Journal of Veterinary Internal Medicine 30(5) pp1564–1574 states that "In dogs and cats with hypercalcemia, correcting or controlling hypercalcemia aids in preventing calcium oxalate urolith recurrence. Doing so is difficult in cats with idiopathic hypercalcemia and no single treatment has been shown to be effective, including glucocorticoids, bisphosphonate administration, or dietary modification using a high-fiber diet with potassium citrate administration, but 5 cats with idiopathic hypercalcemia had normalization of blood calcium concentrations when treated with a high-fiber diet."

However, you do need to be careful when choosing a fibre. One type of fibre called fructooligosaccharides (FOS) may actually cause elevated calcium levels. See Nutritional Requirements for more information on fibre.

One food to discuss with your vet is Hill's w/d, which has added fibre. The canned version of this food would be suitable for most CKD cats, with a phosphorus level of 0.67% and a protein level of 39.60%. The phosphorus level in the dry food is a little higher at 0.84%, with a protein level of 40.80%, but this might still be acceptable if your cat's phosphorus levels are under control.

High Calcium Levels: Chia Seeds

One form of fibre to consider is chia seeds, which also contain omega 3 fatty acids. Managing feline idiopathic hypercalcemia with chia seeds (Salvia hispanica L.): s case series (2020) Fantinati M & Priymenko N Frontiers in Veterinary Science 22(7) p421 found that giving chia seeds (2g per day) normalised ionised calcium levels in three cats with idiopathic hypercalcaemia after four weeks. The cats in this study only had slightly elevated calcium levels, and it is not clear whether dietary changes alone might have sufficed.

A couple of Tanya's CKD Support Group members have successfully used chia seeds in their cats with hypercalcaemia. They used the 2g per day suggested in the above study, and mixed the seeds with a little water, then let it stand for about twenty minutes before adding part of the mixture to each of the cat's canned foods over the day. It is probably best to keep the mixture in the fridge between feedings.

High Calcium Levels: Advanced Treatments

These treatments may be necessary if the simple treatments described above  do not resolve the problem. They all have pros and cons, and some of the cons are potentially serious. Discuss with your vet.

• Corticosteroids

• Loop diuretics (furosemide)

• Bisphosphonate drugs: alendronate or pamidronate

• Calcitriol (in rare circumstances)

High Calcium Levels: Corticosteroids

Corticosteroids such as prednisolone may sometimes be used to control high calcium levels. Hypercalcemia in dogs and cats (2016) Peterson ME Merck Veterinary Manual says "Administration of prednisone results in longterm decreases in ionized and total calcium concentrations in some cats." In Pitfalls in the management of idiopathic hypercalcemia in cats (2013) Peterson ME Proceedings of the 2013 American College of Veterinary Internal Medicine (ACVIM) Forum pp538-540, Dr Peterson states "If normocalcemia has not been restored after a dietary feeding trial of 6 to 8 weeks, treatment with glucocorticosteroids should be considered...Approximately 50% of cats with idiopathic hypercalcemia become normocalcemic with 5 or 10 mg prednisone/day."

Using steroids can have certain undesirable side effects. Pitfalls in the management of idiopathic hypercalcemia in cats (2013) Peterson ME Proceedings of the 2013 American College of Veterinary Internal Medicine (ACVIM) Forum pp538-540 says "approximately a third of cats will develop secondary diabetes, especially with long-term, high-dose steroid therapy." See steroids for more information about possible problems with steroids..

High Calcium Levels: Loop Diuretics Such As Furosemide (Lasix))

Furosemide (Lasix) is a loop diuretic. Diuretics are commonly used in heart disease, but may occasionally be used to help reduce high calcium levels, which they do by increasing the excretion of calcium via the kidneys.

Diuretics are not usually appropriate for CKD cats because they may increase the risk of dehydration, but be guided by your vet.

High Calcium Levels: Bisphosphonate Drugs: Alendronate or Pamidronate

If ionised calcium levels remain high despite trying the above treatments, you may have to consider using bisphosphonate drugs. These medications are commonly used to treat humans with osteoporosis, and are occasionally used in hypercalcaemic cats. They help by encouraging the bones to absorb calcium, which then reduces calcium levels in the blood.

Hypercalcemia in chronic kidney disease (2024) van den Broek H International Renal Interest Society states "If serum ionized calcium concentration has not improved 6 weeks after diet change, treatment with specific calcium lowering medications such as bisphosphonates (PO alendronate, IV pamidronate) or prednisolone could be instigated. Bisphosphonates as a treatment option for hypercalcemia have only been examined in few animals, but appear safe and reasonably effective in lowering serum calcium concentrations, although hypercalcemia may reoccur after treatment is stopped."

Alendronate (Fosamax)

One drug in this family is called alendronate (Fosamax). This may be administered intravenously in a hospital setting, though it may also be given orally at home once a week. Treatment of ionized hypercalcemia in 12 cats (2006-2008) using PO-administered alendronate (2015) Hardy BT, de Brito Galvao JF, Green TA, Braudaway SR, DiBartola SP, Lord L & Chew DJ Journal of Veterinary Internal Medicine 29(1) pp200-6 looked at the use of alendronate in cats with idiopathic hypercalcaemia. The cats were given a dose of 5-20 mg orally every seven days. The study reduced calcium levels in all the cats with no major side effects and concludes "Alendronate was well tolerated and decreased iCa in most cats for the 6-month period of observation."

The usual starting dose is 10mg for cats, given orally only once a week, and many people opt for a twelve hour fast beforehand. The cat must stay upright for at least 15 minutes after taking it so as to avoid oesophageal damage. Always follow the treatment with a water chaser. Plumb's suggests buttering the cat's lips to encourage swallowing. Alendronate dosing protocol for cats with idiopathic hypercalcaemia (2014) Peterson ME Insights into Veterinary Endocrinology discusses dosing.

Ionised calcium levels should be checked regularly, starting four weeks after the first dose, and the dose should be adjusted if necessary .

If you are using ranitidine (Zantac 75) intravenously for gastric hyperacidity (highly unlikely outside a hospital setting), be aware that it doubled the effect of alendronate in one human study.

There is some concern about using bisphosphonates in CKD patients. Plumb's Veterinary Drug Handbook states that "because of a lack of experience, the drug is not recommended for use in human patients with severe renal dysfunction." 

Drugs lists possible side effects of alendronate in humans.

Pamidronate

Another bisphosphonate is pamidronate. Uses and effectiveness of pamidronate disodium for treatment of dogs and cats with hypercalcemia (2005) Hostutler RA, Chew DJ, Jaeger JQ, Klein S, Henderson D, DiBartola SP Journal of Veterinary Internal Medicine 19(1) pp29-33 found it appeared to be safe and effective.

Use of bisphosphonates to treat severe idiopathic hypercalcaemia in a young Ragdoll cat (2011) Whitney JL, Barrs VR, Wilkinson MR, Briscoe KA & Beatty JA Journal of Feline Medicine & Surgery 13(2) pp129-34 discusses the case of a young cat with idiopathic hypercalcaemia who became clinically normal following the use of bisphosphonate drugs. Idiopathic means no obvious cause could be found.

However, pamidronate has to be given intravenously and may cause electrolyte imbalances. Like alendronate, it may be contraindicated for CKD cats.

High Calcium Levels: Calcitriol

Calcitriol is a hormone produced by the kidneys which helps to regulate parathyroid hormone (PTH). Some people use calcitriol as a supplement to try to control secondary hyperparathyroidism.

Although calcitriol may cause calcium levels to rise, in some cases it may actually be used to try to reduce calcium levels. However, this should only be attempted if ionised calcium is above midrange (or even at the top of the normal range), and an intermittent dosing schedule should be used. The causes and consequences of feline hypercalcemia (2009) Cook AK Presentation to the ACVIM Forum explains more about treatments for hypercalcaemia and mentions the use of calcitriol in CKD cats who have both elevated calcium and elevated ionised calcium levels. It states that if ionised calcium levels increase while using calcitriol for this purpose, it must be discontinued immediately.

I would definitely try other treatment options first.

See below for more information on calcitriol.

High Calcium Levels: Cancer

Although it is fairly rare, high calcium levels may be caused by cancer, so if the cause of your cat's hypercalcaemia is unknown, especially if you find the treatments above ineffective, it is worth asking your vet to test for cancer. The causes and consequences of feline hypercalcemia (2009) Cook AK Presentation to the ACVIM Forum explains more about this.

Treating paraneoplastic hypercalcemia in dogs and cats (2007) Fan TM, de Lorimier L-P, Lucas P & Lacoste H Veterinary Medicine May 2007 discusses possible treatment options in this situation (many of which are the same as those discussed above).

Elevated PTH and Secondary Hyperparathyroidism

Untreated elevated parathyroid hormone (PTH) levels may eventually lead to a condition called secondary hyperparathyroidism.

These are the treatments which may be used:

• Phosphorus control

• Magnesium supplementation

• Famotidine (Pepcid AC)

• Calcitriol

• Calcimimetics: Cinacalcet (Sensipar)

Secondary Hyperparathyroidism: Phosphorus Control

The first line of attack to reduce the risks of secondary hyperparathyroidism is to feed your CKD cat a diet low in phosphorus. If this is not sufficient, you should also use phosphorus binders.

For many cats, phosphorus control will be enough to avoid secondary hyperparathyroidism. Survival of cats with naturally occurring chronic renal failure: effect of dietary management (2000) Elliott J, Rawlings JM, Markwell PJ, Barber PJ Journal of Small Animal Practice 41(6) pp235-242 found that a low phosphorus diet, with added binders where necessary, prevented the rise in parathyroid hormone levels seen in the control cats whose phosphorus levels were not restricted. In fact, the cats who ate reduced phosphorus food or food with added phosphorus binders lived more than twice as long as those who did not.

Phosphorus control is such an important topic for CKD cats that there is a page devoted to the subject here, and a page devoted to phosphorus binders (which are used when dietary restriction of phosphorus is not sufficient to control phosphorus levels) here.

Magnesium Supplements

Secondary Hyperparathyroidism: Famotidine

Famotidine (Pepcid AC) is commonly used in CKD cats to block the production of gastric hyperacidity.

As a side effect it may reduce PTH levels in CKD patients. Famotidine reduces serum parathyroid hormone levels in uremic patients (1991) Arik N, Arinsoy T, Sayín M, Taşdemir I, Yasavul U, Turgan C, Caglar S Nephron 59(2) p333 explains more about this, but I am not aware of any detailed, controlled studies into this. Therefore I would not use famotidine to treat elevated PTH levels only, but if you are using it anyway to control gastric hyperacidity, you may possibly see a reduction in PTH levels as well.

Secondary Hyperparathyroidism: Calcitriol

• What is calcitriol?

• What does it do?

• Why the use of calcitriol in cats is debated

• Considerations before starting calcitriol

• When to start

• Dosage

• How to give

• Monitoring

• Side effects and interactions

• Where to buy

Calcitriol: What is It?

Calcitriol is the active form of vitamin D. Despite its name, it is not the same as the vitamin D we eat or obtain from sunlight or supplements. Calcitriol is actually a hormone, and it plays an important part in regulating phosphorus and calcium levels in the body.

Here is how calcitriol is produced in the body:

• Vitamin D2 (ergocalciferol) is found in some plant-based foods. 

• Vitamin D3 (cholecalciferol) is produced in the skin when the skin is exposed to sunlight (which is a poor mechanism in cats, probably because of their fur coats), but may also be found in certain foods such as eggs.

• Supplements containing vitamin D2 and vitamin D3 are also widely available (but not normally required in cats).

• Vitamin D (whether D2 or D3) is converted in the liver into calcidiol (25-hydroxyvitamin D [25(OH)D], or 25-hydroxycholecalciferol).

• Calcidiol is then converted in the kidneys into the biologically active form known as calcitriol (1,25-dihydroxyvitamin D [1,25(OH)2D], or 1,25 dihydroxycholecalciferol.

Calcitriol: What Does It Do?

Calcitriol facilitates the absorption of calcium from the gastointestinal tract and the release of calcium from the bones where it is stored.

In healthy cats, if phosphorus levels are too high, or if calcium levels are too low, the levels of ionised calcium in the body fall. The parathyroid glands are then stimulated to produce more parathyroid hormone, which tries to adjust the levels of calcium and phosphorus to their correct levels, partly by taking calcium from the bone and partly by increasing the excretion of phosphorus in the urine. Parathyroid hormone also stimulates the kidneys to produce calcitriol, which helps to increase the levels of ionised calcium in the blood.

Once ionised calcium has been restored to the correct level, this process should cease because normally calcitriol can stop the secretion of PTH. Unfortunately, as kidney function reduces and calcitriol is not produced in adequate amounts, this mechanism may no longer work properly. Therefore, phosphorus levels may continue to rise and may also block calcitriol synthesis. A vicious circle then results, and eventually a condition called secondary hyperparathyroidism may develop.

Human patients with secondary hyperparathyroidism are therefore sometimes given additional calcitriol, and some people do the same for their CKD cats. The role of phosphorus in feline chronic renal disease (2010) Chew D & Kidder A CVC in San Diego Proceedings states "In some instances, PTH cannot be controlled despite dietary intervention and use of intestinal phosphate binders. Other treatments with calcitriol and calcimimetics may be indicated in these cases."

Mar Vista Vet has more information on calcitriol.

Calcitriol Debate

The use of calcitriol in cats is somewhat controversial, in part because no studies clearly show that it is effective for cats (although it does appear to be effective for dogs). In fact, Announcement of changes to IRIS guidelines (2023) International Renal Interest Society mentions one of the changes to be "The removal of the mention of calcitriol treatment for cats in the management of Stage 3 and 4 CKD due to lack of any evidence of a beneficial effect."

The willingness or otherwise of US vets to use calcitriol may depend in part upon which vet school they attended. Initial studies at Cornell University College of Veterinary Medicine and later studies at the University of Georgia College of Veterinary Medicine found that using calcitriol caused calcium levels to rise. However, Ohio State University College of Veterinary Medicine has also conducted research into the use of calcitriol in CKD cats, and it believes that the risk of hypercalcaemia is dosage dependent and that the risk can be managed.

Calcitriol: Pro

Calcitriol advocates believe that it is possible to have normal phosphorus levels and still have secondary hyperparathyroidism. They believe not only that the use of calcitriol is therefore essential, but also that calcitriol may help control uraemia and even slow the progression of CKD.

Benefits of calcitriol therapy and serum phosphorus control in cats and dogs with chronic renal failure. Both are essential to prevent or suppress toxic hyperparathyroidism (1996) Nagode LA, Chew DJ, Podell M Veterinary Clinics of North American Small Animal Practice 26 pp1293-1330 discusses the importance of controlling secondary hyperparathyroidism in cats and the role of calcitriol in doing so.

However, Comparison of the effects of daily and intermittent-dose calcitriol on serum parathyroid hormone and ionized calcium concentrations in normal cats and cats with chronic kidney failure (2006) Hostutler RA, DiBartola SP, Chew DJ, Nagode LA, Schenck PA, Rajala-Schultz PJ, Drost WT Journal of Veterinary Internal Medicine 20(6) pp1307-13 found "At the dosages used, calcitriol treatment did not result in significant differences in serum parathyroid hormone concentrations before and after treatment in both normal cats and cats with chronic renal failure. With these dosages, adverse affects of calcitriol administration were not seen. Potential reasons for lack of apparent effect include small sample size, insufficient duration of study, insufficient dosage of calcitriol, problems with formulation or administration of calcitriol, and variable gastrointestinal absorption of calcitriol.”

Calcitriol, calcidiol, parathyroid hormone, and fibroblast growth factor-23 interactions in chronic kidney disease (2013) de Brito Galvao JF, Nagode LA, Schenck PA & Chew DJ Journal of Veterinary Emergency and Critical Care 23(2) pp 134–162 is a more recent paper which discusses calcitriol and its effectiveness  and renoprotective qualities for dogs and humans, but which concludes "Long-enough trials to determine any benefits for CKD cats treated with calcitriol have yet to be conducted."

Calcitriol: Con

There is evidence that you can control PTH levels and prevent, reverse or at least postpone the development of secondary hyperparathyroidism simply by controlling phosphorus levels. Feline hyperparathyroidism: pathophysiology, diagnosis and treatment of primary and secondary disease (2015) Parker VJ, Gilor C & Chew DJ Journal of Feline Medicine and Surgery 17(5) pp427-439 says “Treatment of RSHP includes controlling serum phosphorus concentrations by feeding a reduced phosphorus diet ± administration of a dietary phosphorus binder.”  See above for information on how to do this.

Chronic kidney disease (CKD) in dogs and cats — staging and management strategies (2015) Chew D Presentation to the Virginia Veterinary Medical Association 2015 Virginia Veterinary Conference says "Early phosphorus restriction in CRF has been shown in dogs and cats to blunt or reverse renal secondary hyperparathyroidism. In a study of cats with naturally-occurring CRF, renal secondary hyperparathyroidism was successfully managed by dietary restriction of phosphorus; one-third of the cats also required treatment with phosphorus binders.”

There is also some concern about the potential for calcitriol supplementation to cause both elevated phosphorus levels and elevated calcium levels. Renal Disease (2006) Polzin DJ Delaware Valley Academy of Veterinary Medicine states "the decision to use calcitriol must be made with caution because hypercalcemia is a potentially serious complication. Sustained calcitriol-induced hypercalcemia will likely result in reversible or irreversible reduction in GFR." GFR is a measure of kidney function.

Renal Disease (2006) Polzin DJ Delaware Valley Academy of Veterinary Medicine states "Nagode and colleagues have suggested that normalization of PTH levels using calcitriol therapy may provide clinical benefits that cannot be achieved by phosphorus restriction alone including amelioration of many clinical signs associated with CKD. We have been unable to completely substantiate these claims, but did find that calcitriol therapy significantly prolonged survival in dogs with stages 3 and 4 CKD."  He further states "A recommendation for or against routine use of calcitriol awaits results of properly designed controlled clinical trials."

What do I think? I've been running this website for 25 years and I have seen very few cats use calcitriol during that time. People who have given it to their cats seem overall to be happy with it, but there is no evidence of its effectiveness in CKD cats, and it is tricky to obtain in cat-sized dosages. Personally, I would save my money and efforts and focus on more proven treatments.

Calcitriol: Considerations Before Starting

Calcitriol will not work for cats whose phosphorus levels are already above 8 mg/dl or 2.6 mmol/L, and is of limited value for cats with phosphorus levels above 6 mg/dl or 1.9 mmol/L.

Calcitriol, calcidiol, parathyroid hormone, and fibroblast growth factor-23 interactions in chronic kidney disease (2013) de Brito Galvao JF, Nagode LA, Schenck PA & Chew DJ Journal of Veterinary Emergency and Critical Care 23(2) pp 134–162 says “Serum phosphorus should be less than 1.9 mmol/L [<6.0 mg/dL] before starting calcitriol treatment and thereafter to ensure calcitriol efficacy in PTH suppression."

Therefore, if your cat's phosphorus levels are above 6 mg/dl or 1.9 mmol/L, you must take all possible steps to control phosphorus levels (see The Importance of Phosphorus Control for information on how to do this) before using calcitriol.

Calcitriol may cause ionised calcium levels to increase. Prolonging life and kidney function (2007) Chew D Presentation to the 32nd World Small Animal Veterinary Association World Congress explains more about this.

On the other hand, Chronic kidney disease (CKD) in dogs and cats — staging and management strategies (2015) Chew D Presentation to the Virginia Veterinary Medical Association 2015 Virginia Veterinary Conference mentions that calcitriol will be less effective if your cat's ionised calcium level is low.

You also need to consider your cat's total calcium levels. If your cat's calcium level x phosphorus level is over 60-70 in US values or over 5 in international values, your cat is at risk of soft tissue mineralisation, and since calcitriol may increase calcium levels, you should not use it until this level has been reduced. Chronic kidney disease (CKD) in dogs and cats — staging and management strategies (2015) Chew D Presentation to the Virginia Veterinary Medical Association 2015 Virginia Veterinary Conference states: "Calcitriol should not be administered until hyperphosphatemia has been controlled. If the Ca X P solubility product exceeds 60-70, calcitriol should be avoided because of the risk of soft-tissue mineralization." This also applies if you multiply phosphorus by ionised calcium, and the level is higher than 8.75 in US values or 35 in international values.

Calcitriol: When to Start

As stated earlier, calcitriol is not an essential treatment, so if you are unable to obtain it or cannot afford it, or if it does not seem to agree with your cat, I would give it a miss.

Please also see considerations before starting.

If you and your vet do decide to use calcitriol, and wish to start before your cat's PTH levels are elevated, Dr Larry Nagode, formerly of Ohio State University College of Veterinary Medicine, now sadly deceased, advises starting calcitriol when creatinine reaches 2 mg/dl or 175 mmol/L, assuming pre- and post-renal causes of the elevated creatinine have been ruled out.

Cinacalcet (Sensipar)

Cinacalcet (Sensipar) was approved by the US Food and Drug Administration in March 2004 for the treatment of secondary hyperparathyroidism in humans. Cinacalcet hydrochloride (Sensipar) (2005) Poon G Baylor University Medical Center Proceedings 18(2) pp182-184 gives an overview of cinacalcet.

Health Canada reports that in 2007 the use of cinacalcet in humans was restricted to those on dialysis because it was found that patients receiving cinacalcet who were not on dialysis were more likely to develop low calcium levels.

Cinacalcet is occasionally used in CKD cats for whom other attempts to reduce parathyroid hormone levels have not worked. The role of phosphorus in feline chronic renal disease (2010) Chew D & Kidder A CVC in San Diego Proceedings states "In some instances, PTH cannot be controlled despite dietary intervention and use of intestinal phosphate binders. Other treatments with calcitriol and calcimimetics may be indicated in these cases."

Cinacalcet works by lowering calcium levels, so should not be used in patients with low calcium levels. It is very expensive. I have only heard of a couple of people who have used it in cats, and never heard back to see how they got on with it.

Elevated PTH levels can contribute to anaemia. Improved parathyroid hormone control by cinacalcet is associated with reduction on darbepoetin requirement in patients with end-stage renal disease (2011) Battistella M, Richardson RM, Bargman JM & Chan CT Clinical Nephrology 76(2) pp99-103 found that cinacalcet meant less ESA treatment for anaemia was required in human patients. I don't think this is unique to cinacalcet, but rather because of the reduction in PTH levels.

Amgen — prescribing information from the manufacturer.

MedlinePlus explains more about cinacalcet.

Etelcalcetide

Etelcalcetide is another member of the calcimimetic family which is marketed under the name Parsabiv. It was approved in the USA in 2017 for the treatment of secondary hyperparathyroidism in humans on dialysis. I am not aware of any usage in cats to date.

Amgen - prescribing information from the manufacturer.