Low Phosphorus Levels

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Ionised Calcium Levels

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Ionised Calcium Levels

Phosphorus Multiplied by Total Calcium

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Home > Diagnosis > Phosphorus, Calcium, PTH and Secondary Hyperparathyroidism



  • There is a balance in the body between phosphorus and calcium levels.

  • In CKD cats, imbalances are very common.

  • In the worst case, problems then also arise with a hormone called parathyroid hormone (PTH).

  • These imbalances can eventually lead to a condition called secondary hyperparathyroidism, which can have serious effects on the cat.

  • It is therefore important to monitor phosphorus, calcium and PTH levels in all CKD cats.

  • Click on the links at the top of the sidebar to the left to view the contents of this page.

Phosphorus (P or Pi)


Phosphorus is a mineral which is essential for bodily function. Most of the body's phosphorus is contained in bone.


Phosphorus is important for energy production, acid base balance (imbalances in this area can lead to a condition known as metabolic acidosis) and for delivering oxygen to the body's cells.


The kidneys are responsible for removing excess phosphorus from the body, but damaged CKD kidneys find this more difficult, so phosphorus levels within the body begin to rise. This activates a phosphatonin called fibroblast growth factor 23 (FGF-23), which is important for the regulation of phosphorus levels in the blood. FGF-23 may play a role in the early detection of CKD, as discussed in Early Detection.


If you are looking at your cat's blood test results, phosphorus may be shown as phos, P or Pi.


Oregon State University provides an overview of phosphorus and its functions in the body.


Cornell University College of Veterinary Medicine provides an overview of phosphorus.

Low Phosphorus Levels (Hypophosphataemia)

Low phosphorus levels are extremely rare in CKD cats, but may occasionally arise. In most cases they tend to happen if you are over zealous with phosphorus binders used to control high phosphorus levels, but may also be seen in cats with diabetes controlled by insulin.


Certain types of cancer may cause low phosphorus levels, as may feeding a cat following a period of prolonged starvation. Refeeding syndrome Sumner C Angell Medical Center explains more about this.


Low phosphorus levels are usually not a problem, although they may sometimes cause lethargy or anaemia, and if the level falls really low, seizures might occur.

  • If it goes below around 1.5 mg/dl (0.5 mmol/L international), it is potentially life threatening.

  • I would aim for a level no lower than 3 mg/dl US (1.0 mmol/L international).

Pet MD has some information about low phosphorus levels.


High Phosphorus Levels (Hyperphosphataemia)

Virtually every CKD cat will eventually develop high phosphorus levels (hyperphosphataemia). This happens because the damaged kidneys can no longer excrete excess phosphorus properly.


There is a balance in the body between phosphorus and calcium. Thus elevated phosphorus levels can adversely affect calcium levels, with potentially serious consequences, and can generally make the CKD progress faster. They also make the cat feel very unwell. Symptoms include nausea, weakness and loss of appetite.


Because of these risks, having a phosphorus level within the normal range is not sufficient for a CKD cat.

  • If your cat's phosphorus level is over 6 mg/dl (USA) or 1.9 mmol/L (international), you need to take steps to control it.

  • Your aim is to reduce your cat's phosphorus to a level of below 4.6 mg/dl (1.5 mmol/L international).

  • You may not be able to get it this low if your cat is in IRIS Stage 3 or 4, but aim never to let it go above 6 mg/dl (1.9 mmol/L international).

Since phosphorus control is so important, there are separate pages All About Phosphorus and Phosphorus Binders.




Calcium is a mineral which is also found primarily in bone. It is important for nerve function, muscle contraction, blood clotting, and the skeleton.


Oregon State University provides an overview of calcium and its functions in the body.


Cornell University College of Veterinary Medicine provides an overview of calcium.


Calcium Forms


Calcium exists in three different forms in the blood:

  • bound, which is bound to proteins (around 80% to albumin and 20% to globulins)

  • complexed, with certain anions such as bicarbonate

  • ionised, or free

The total calcium figure on a blood chemistry panel is the total of all of these, with ionised calcium comprising about 50% of the total. Ionised calcium is the value of main concern for most CKD cats.


CKD cats may have reduced calcium levels (hypocalcaemia) or elevated calcium levels (hypercalcaemia). Whether these are a concern depends in part upon the ionised calcium level.

Low Calcium Levels (Hypocalcaemia)


Calcitriol (1,25 dihydroxycholecalciferol) is the active form of vitamin D3, which, despite its name, is actually a hormone. It facilitates the absorption of calcium from the gastointestinal tract and the release of calcium from the bones where it is stored. Cats have to make calcitriol from vitamin D before they are able to use it, and the final step of this process is performed by the kidneys. 


Because of their failing kidneys, CKD cats may no longer be able to convert vitamin D into calcitriol, so they absorb less calcium from the gastrointestinal tract and release less from the bones, and thus levels of calcium in the body may fall.


Hypocalcaemia is generally considered to be present when total calcium is below 9 mg/dl or ionised calcium is below 1.2 mmol/L.


Excess phosphorus levels may also reduce calcium levels by suppressing the production of calcitriol.


Up to 50% of cats with pancreatitis have hypocalcaemia. Serum ionised calcium as a prognostic risk factor in the clinical course of pancreatitis in cats (2015) Dias C & Carreira LM Journal of Feline Medicine & Surgery 17(12) pp984-90 states "The results suggest that hypocalcaemia is common in patients with pancreatitis and that [Ca(2+)i] may be used as a prognostic risk factor for predicting the clinical course of the disease, with values < 1 mmol/l corresponding to a poor prognosis."


VCA Animal Hospitals discusses hypocalcaemia.


Colorado State University has some information about calcitriol.


Cornell University College of Veterinary Medicine has some information about low calcium levels (scroll down)


High Calcium Levels (Hypercalcaemia)


Elevated calcium levels are relatively common in CKD cats. Hypercalcemia in cats (2001) Chew D Presentation to the World Small Animal Veterinary Association World Congress states that 38% of hypercalcaemic cats in one study had CKD.


The range for calcium varies from laboratory to laboratory, but generally speaking:

  • a level over 11 mg/dl US (2.75  mmol/L international) is considered to be hypercalcaemia, though you may not see any symptoms until it is higher than this.

  • If calcium goes above 15 mg/dl US (3.75 mmol/L international), a cat may have constipation, little appetite and sleep a lot.

  • if levels approach 20 mg/dl US (5.0 mmol/L international), the cat may go into a coma and die. Hypercalcemia in dogs and cats (2016) Peterson ME Merck Veterinary Manual says "serum concentrations of >18 mg/dL are often associated with severe, life-threatening signs." Fortunately, such extremely high levels are very rare, so in practice you are unlikely to see them.

Even if your cat's calcium levels are not high enough to be lifethreatening, they can still have adverse effects on the cat's body. Hypercalcemia in chronic kidney disease (2018) Van den Broek H International Renal Interest Society states "hypercalcemia may worsen signs related to CKD as it further reduces glomerular filtration rate (GFR) and can cause anorexia, polyuria and polydipsia, muscle weakness, and constipation. Hypercalcemia can furthermore contribute to urolithiasis and, especially in combination with hyperphosphatemia, to soft tissue calcification. Calcification of the aorta, gastric wall, kidneys, and paws has been reported in cats with CKD."


Using urine specific gravity (2015) Watson ADJ, Lefebvre HP & Elliott J International Renal Interest Society mentions that cats with hypercalcaemia may also have a low urinary specific gravity (USG).


Guidelines for evaluating hypercalcemic cats (2008) Cook AK Veterinary Medicine explains more about diagnosing hypercalcaemia.


Clinical approach to the hypercalcemic patient (2008) Ward C CVC in Washington Proceedings also discusses diagnosis.


Hypercalcemia in dogs and cats: etiology and diagnostic approach (2002) Nelson R Presentation to the 27th World Small Animal Veterinary Association Congress 2002 explains more about hypercalcaemia. 


High Calcium Levels: Causes

The normal level of calcium in the blood is in ratio to the phosphorus level - calcium levels are usually at an approximate level of 100 - 200% of phosphorus levels. Since phosphorus levels are often high in CKD cats, calcium levels may also rise in an attempt to maintain the ratio. Conversely, if elevated phosphorus levels are reduced, often the calcium levels will also reduce to a satisfactory level. 


Other causes of hypercalcaemia include metabolic acidosis (since acid in the blood leads to the release of calcium from proteins or from bone) or, rarely, cancer.


Around 15% of cats with hypercalcaemia have calcium oxalate stones in the urine, though it is not clear which comes first.


Hyperthyroidism may be another factor.


A type of fibre called fructooligosaccharides (FOS) may cause elevated calcium levels.


Excess vitamin D in the diet causes hypercalcaemia. In 2006, Royal Canin had to recall some foods to which too much vitamin D had been added by mistake, causing hypercalcaemia in four cats. Many commercial foods seem to contain levels in excess of current maximum US allowances (10,000 iu/kg for adult cats). Update on the etiology of tooth resorption in domestic cats (2005) Reiter AM, Lewis JR & Okuda A Veterinary Clinics Small Animal Practice 35 pp913-942 states "results of experimental studies on cats fed diets high in vitamin D3 (15,000–33,840 IU/kg of dry matter) were contradictory, ranging from no evidence of detrimental effects on feline health to a high prevalence of renal dysfunction and mortality."


Some people have found that using lactulose to control constipation has led to hypercalcaemia in their cats. This might just be coincidence, but you may wish to avoid lactulose if your cat already has hypercalcaemia, and consider alternative constipation treatments if your cat develops hypercalcaemia while using lactulose. Slippery elm bark also contains calcium, so it is probably safer not to use it if your cat has hypercalcaemia.


Top 10 differentials for hypercalcemia (2012) Peterson ME Insights Into Veterinary Endocrinology discusses how to narrow down the cause in dogs using mnemonics (GOSH DARN IT and HARD IONS).


Hypercalcemia - distinguishing causes and preventing complications (2008) Mooney CT Presentation to the World Small Animal Veterinary Association World Congress discusses possible causes of hypercalcaemia.


Michigan State University Diagnostic Center for Population & Animal Health has some information about hypercalcaemia.


High Calcium Levels: Idiopathic Hypercalcaemia

In many cases, no cause for the elevated calcium levels can be found - this is known as idiopathic hypercalcaemia.


Michigan State University Diagnostic Center for Population & Animal Health explains more about idiopathic hypercalcaemia.


What's causing  idiopathic hypercalcemia in cats (2012) Peterson ME Insights Into Veterinary Endocrinology discusses idiopathic hypercalcaemia.


High Calcium Levels: Initial Steps

The first thing to do is to run the calcium test again. Hypercalcemia in dogs and cats: etiology and diagnostic approach (2002) Nelson R Presentation to the 27th World Small Animal Veterinary Association Congress 2002 states "Hypercalcemia should always be reconfirmed, preferably from a nonlipemic blood sample obtained from the dog or cat following a 12 hour fast, before embarking on an extensive diagnostic evaluation."


If the second test also indicates hypercalcaemia for which no obvious cause can be found, the next step is to run an ionised calcium test. If this is normal, there is no need to worry about the hypercalcaemia per se. However, you do need to consider the product of total calcium multiplied by phosphorus (see below).


Ionised Calcium


Ionised calcium is the amount of free calcium, i.e.  metabolically active calcium, in the body. It forms about 50% of total calcium.


Many vets simply look at the total calcium levels in blood test results, but these are not an accurate indication of ionised calcium levels. Prediction of serum ionized calcium concentration by serum total calcium measurement in cats (2010) Schenck PA & Chew D Canadian Journal of Veterinary Reseach 74(3) pp209–213 states "In general, tCa concentrations cannot be relied on to accurately assess calcium status in cats as shown by discordance with iCa measurement. This is especially important in cats with CRF where abnormalities in TP, albumin, and complexed calcium fractions may be present. Due to the high number of false negatives, many cats may appear normocalcemic when tCa is measured, but actually have a derangement in calcium homeostasis. Ionized calcium concentration must be directly measured to accurately assess calcium status.”


Therefore, if your cat has high calcium levels for which no obvious cause can be found, ask your vet to check ionised calcium, which will almost always have to be done by an external laboratory (although not all laboratories can do this).


Michigan State University Diagnostic Center for Population & Animal Health gives a reference range for ionised calcium of 1.0-1.4 mmol/l.

  • If ionised calcium levels are normal (or even low, which is not unusual in CKD cats), there is no need to treat for hypercalcaemia in a CKD cat.

  • If ionised levels are elevated, you do need to take action because if this is the case for any length of time, it may contribute to CKD. See Treatments for how to do this.

Cornell University College of Veterinary Medicine has some information about ionised calcium levels.


Michigan State University Diagnostic Center for Population & Animal Health explains more about ionised calcium, and can perform the ionised calcium test (see below for how to


Phosphorus Multiplied by Total Calcium


Although as stated above, ionised calcium is usually what matters for CKD cats, you do have to take total calcium levels into account in one main regard: you have to consider the level of phosphorus multiplied by total calcium.


This is because cats with a high number are at risk of a problem called calcification. Hypercalcemia in cats (2001) Chew D Presentation to the World Small Animal Veterinary Association World Congress says "The degree of interaction with serum phosphorus is important, as those with a total serum calcium times phosphorus concentration product greater than 70 are most likely to have severe tissue changes associated with mineralization."


As a rough guide, if phosphorus multiplied by total calcium is higher than around 60 in US values or 5 in international values, your cat is at risk.


Alternatively, you can multiply phosphorus by ionised calcium, and if the level is higher than 8.75 in US values or 35 in international values, there is a risk of calcification.


It is less likely to be a problem if the product is above these levels because of a higher calcium level rather than a higher phosphorus level; but I would still recommend trying to reduce these levels if at all possible.


Parathyroid Hormone (PTH) and Calcitriol


Phosphorus and calcium levels in the body are controlled by glands called the parathyroid glands, which adjust phosphorus and/or calcium levels as appropriate via two hormones called:

  • parathyroid hormone (also known as PTH) and

  • calcitriol

Calcitriol (1,25 dihydroxycholecalciferol) is the active form of vitamin D3, which, despite its name, is actually a hormone. It facilitates the absorption of calcium from the gastointestinal tract and the release of calcium from the bones where it is stored. Cats have to make calcitriol from vitamin D before they are able to use it, and the final step of this process is performed by the kidneys. 


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.


High levels of PTH may also adversely affect the production of erythropoietin, a hormone which stimulates the bone marrow to make red blood cells, and may therefore be a factor in a CKD cat developing anaemia.  Role of secondary hyperparathyroidism in erythropoietin resistance of chronic renal failure patients (2002) Drücke TB & Eckardt K-U Nephrology Dialysis Transplantation 17 Supp.5 pp17-21 mentions this in human patients.


Many cats who are only in IRIS Stage 2 will have normal phosphorus levels but will still have elevated PTH levels, which can still lead to secondary hyperparathyroidism. Parathyroid hormone concentration in geriatric cats with various degrees of renal function (2012) Finch NC, Syme HM & Elliott J Journal of the American Veterinary Medical Association 241(10) pp1326-35 found that "renal secondary hyperparathyroidism can develop prior to azotemia in cats, even in the absence of hyperphosphatemia and hypocalcemia."


Generally speaking, problems tend to arise when PTH is around three times the level of the top of the normal range. Michigan State University Diagnostic Center for Population & Animal Health states that the reference range for PTH is 0.4 - 2.5 pmol/L. Therefore toxicity can be expected when PTH is 7.5 pmol/L or over. However, if it is at all elevated, it is best to take action to reduce it as soon as possible, so as to avoid secondary hyperparathyroidism.


Calcium and phosphate homeostasis in hyperthyroid cats: associations with development of azotaemia and survival time (2012) Williams TL, Elliott J & Syme HM Journal of Small Animal Practice 53(10) pp561-71 found that approximately 60% of the hyperthyroid cats in the study had elevated parathyroid hormone levels, which decreased following treatment of the hyperthyroidism in cats who did not have CKD.


The US Food and Drug Administration (2017) warns that in humans, taking large doses of biotin supplements may adverselty affect test results for T4, T3 and PTH.


Oregon State University has a helpful diagram showing calcium and phosphorus homeostasis.


Cornell University College of Veterinary Medicine has some information about parathyroid hormone.


Parathyroid hormone, a uremic toxin (2009) Rodriguez M & Lorenzo V Seminars in Dialysis 22(4) pp363-8 mentions that PTH can be elevated in human patients and can have a variety of effects.


Secondary Hyperparathyroidism (RSHP, formerly 2-HPTH)


Secondary hyperparathyroidism is caused by part of the body's mechanism for controlling phosphorus and calcium levels effectively going into overdrive, as explained above.  It is not the same as hyperthyroidism.


As problems with the regulation of calcitriol and parathyroid stimulating hormone (PTH) progress, levels of PTH will continue to rise and eventually secondary hyperparathyroidism results. Blood calcium levels may appear to be normal or low but over time various symptoms may appear, including lack of appetite, anaemia, reduced immunity to infection and muscle weakness. Other severe consequences may also result, see below.


Feline chronic renal failure: calcium homeostasis in 80 cases diagnosed between 1992 and 1995 (1998) Barber PJ, Elliott J Journal of Small Animal Practice 39 pp108-116) found that 84% of the CKD cats in the study eventually developed secondary hyperparathyroidism, so it is important to be aware of the possibility.


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 that it is possible for a CKD cat to develop secondary hyperparathyroidism even if phosphorus levels and ionised calcium levels are normal. He explains "In the early stages of chronic kidney disease increased levels of parathyroid hormone (PTH) keep serum phosphorous within the normal range by increasing phosphate excretion into urine. This allows for normalization of serum phosphorous at the expense of hyperparathyroidism."


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 discusses secondary hyperparathyroidism (in very technical terms).


Veterinary Partner explains more about secondary hyperparathyroidism in easy to understand language.


PetCoach has helpful information about this disease.


Endocrine Web is a human site but has a lot of information.


The clinical consequences of secondary hyperparathyroidism: focus on clinical outcomes (2004) Hörl WH Nephrology Dialysis Transplantation 19 (suppl 5) v2-v8 discusses the problems that may arise with secondary hyperparathyroidism in humans.



As secondary hyperparathyroidism progresses, the cat is at risk of tissue calcification, which means that normally soft tissue in the body becomes hardened, which is very painful and can adversely affect proper function. If calcification occurs in the kidneys, kidney function is further worsened and a vicious circle results.


See above for when this becomes a concern.


Kidney Research UK discusses calcification.


Osteodystrophy (Rubber Jaw)

Cats with secondary hyperparathyroidism can also develop problems with their jaw whereby the jawbone softens. This is known as osteodystrophy or colloquially as "rubber jaw."


Chronic kidney disease in cats and the risk of total hypercalcaemia (2017) van den Broek DHN, Chang Y-M, Elliott J & Jepson RE Journal of Veterinary Internal Medicine 31(2) pp465–475 says "One syndrome with abnormalities correlated with CKD and renal osteodystrophy is called chronic kidney disease-mineral and bone disorders (CKD-MBD). Development of this syndrome is a consequence of reduced glomerular filtration rate (GFR) and associated phosphate retention."


University of Georgia College of Veterinary Medicine has a photo of a dog with osteodystrophy.


Oral lesions associated with renal secondary hyperparathyroidism in an English bulldog (2008) Headley SA, Veltrini VC, Fuck EJ, Fuck ET, Curti CE, Bettini CM, Bracarense APFRL, dos Reis ACF Presentation to the Semina Ciências Agrárias 29(n2) pp407-412 has x-rays and photos of a dog with osteodystrophy, although the large lesions seen in this poor dog are unusual.


Medicine Net has an article about renal osteodystrophy in humans.


Testing PTH and Ionised Calcium


Any laboratory can measure phosphorus and total calcium levels, but it is a lot harder to find a laboratory which can measure PTH and ionised calcium.


Michigan State University Diagnostic Center for Population & Animal Health explains why cats with CKD should have PTH and ionised calcium checked.


USA and Canada

Michigan State University Diagnostic Center for Population & Animal Health is the best place to have PTH and ionised calcium levels checked in the USA. Although other US laboratories do offer these tests, they often send them to MSU anyway, so it takes longer and you will pay more. If your vet does not already have an account with MSU, s/he can create one here.


The test is run five days a week (Mon-Fri), and turnaround time is 1-4 days. The sample needs to be sent overnight as a chilled or frozen sample. There is more information here.


Your cat should fast for eight hours before the test in order to avoid lipaemia which may adversely affect the results. The blood can be taken by your own vet (who will of course charge for his/her time and work), who can then send the sample direct to MSU for analysis, using this test form, which also includes guidelines for how to take and send the sample.


As at February 2018, MSU charged US$42 to run both tests, but you can check the current price here (look under Endocrinology, no. 20033).


For people within the USA, MSU can also provide your vet with an insulated prepaid UPS overnight mailing envelope which costs US$19 using this order form (no. 99220). You or your vet will need to provide cold packs to place around the sample (though I heard from one person who said it was included in the package they received from MSU), but overall this usually works out cheaper than paying for chilled overnight shipping yourself.



Cambridge Specialist Laboratory Services can measure PTH and ionised calcium in the UK. There is more information and a price list here. I don't know anybody who has used them yet.


I have heard of British vets submitting samples to MSU, I'm not quite sure how they keep the samples cool enough to last for the length of the journey..


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This page last updated: 07 April 2018

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I have tried very hard to ensure that the information provided in this website is accurate, but I am NOT a vet, just an ordinary person who has lived through CKD with three cats. This website is for educational purposes only, and is not intended to be used to diagnose or treat any cat. Before trying any of the treatments described herein, you MUST consult a qualified veterinarian and obtain professional advice on the correct regimen for your cat and his or her particular requirements; and you should only use any treatments described here with the full knowledge and approval of your vet. No responsibility can be accepted.


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