Feline Idiopathic Hypercalcemia

Mouse – Feline IHC Poster Cat (7/17/99-1/2/18)

Feline Idiopathic Hypercalcemia is an increasing problem rather than just an increasing recognition of an old problem. It is not understood what causes it or why it is becoming prevalent. Many experts believe that acidifying, magnesium-restricted diets lead to IHC in genetically predisposed cats, perhaps due to the calcium resorption from bones. Diet may also be the answer to its management. “Idiopathic (unexplained) Hypercalcemia has been increasingly recognized in cats since 1992. Usually striking young to middle-aged cats, the mild to moderate hypercalcemia has no definable cause following standard diagnostics.” Chew 2001

Many cases of Idiopathic Hypercalcemia are diagnosed almost as a fortuitous side result from pre-surgical or pre-dental blood work, or for another reason such as illness unrelated to the IHC for which the cat has been taken to the veterinarian or a routine blood test. IHC rarely shows symptoms when in the mild to moderate stage. Cats are often asymptomatic especially in the early stages of the disease. Therefore, this condition is typically not clinically recognized, as cats with progressed renal disease usually show a high blood calcium level. It should be noted that kidney failure causes hypercalcemia and hypercalcemia causes kidney failure. It is difficult to determine which came first, but kidney failure tends to progress rapidly in the presence of hypercalcemia. In some instances, hypercalcemia has been associated with calcium oxalate urolithiasis (bladder stones), and many of these affected cats have been fed acidifying diets.

This website is for the owner who has a diagnosed Idiopathic Hypercalcemic cat. I will discuss briefly, the tests and diagnostics associated with this diagnosis and then concentrate on the management of this disease. Although, little research has produced any definitive solutions, diet change and/or additional medications may help to delay or halt its deleterious effects on the cat’s kidneys and other organ systems. With proper management, many cats lead happy normal lives for many years after initial diagnosis.

Tests & Diagnosis

After your cat has been documented with a high serum Total Calcium (tCa) concentration, this test includes ionized calcium, complex calcium, and protein-bound calcium, a test for serum Ionized Calcium (iCa) concentration is required. If the result is normal, the patient is not truly hypercalcemic.

Hypercalcemia Serum Values

tCa > 10 mg/dLandiCa > 5.5 mg/dL or
iCa > 1.4 mmol/L
Mild HCModerate HCSevere HC
tCa < 13 mg/dLtCa 13 – 15 mg/dLtCa > 15 mg/dL
these numbers are for a general idea of hypercalcemic values, results may vary due to differences in laboratory reference ranges

If the iCa is increased, a urinalysis (for evidence of protein, calcium oxalate stones and decreased specific gravity) and serum chemistry profile for renal (kidney) function is evaluated. iCa is affected by exposure to oxygen and pH, so serum samples are collected anaerobically. The sample must also be of a non-lipemic (non-fatty), non-hemolyzed (non-destructed) serum or the results may be skewed. Certain soaps used in the laboratory may also affect the test, so care must be taken in the handling and cleaning of laboratory glassware and equipment that will be utilized for the test. Dehydration is a common clinical situation that can result in mildly increased blood calcium concentration. Blood calcium concentration should be re-evaluated after the patient has been rehydrated by intravenous or subcutaneous (under the skin) administration of fluids.

FeLV (Feline Leukemia Virus) and/or FIV (Feline Immunodeficiency Virus) tests may also be administered in patients with uncertain retroviral status.

Physical examination should prove to be unremarkable but sometimes either through physical examination or radiography, the kidneys in some affected cats are noted as smaller than normal.

If it is possible that your cat was exposed to rat poison, a test for Vitamin D concentration is advocated. Calciferols (Vitamin D) are a type of rodenticide. These compounds work by affecting the levels of Vitamin D and calcium in the body. Ingesting Vitamin D in toxic doses causes hypercalcemia. This is a condition where the calcium level is raised to such a degree that the stomach, kidneys, lungs, blood vessels and heart are all damaged by calcification. If the Vitamin D concentration is normal further tests are warranted.

The serum phosphorus should also be checked as the calcium phosphorus ratio is extremely important in maintaining a healthy feline body. The calcium phosphorus ratio is usually defined as 1.3 : 1 respectively. In rodenticide poisoning, cats often exhibit both high Vitamin D levels (hypervitaminosis D) and high phosphate levels (hyperphosphatemia).

Tests must now be done to determine if your cat has primary hyperparathyroidism (excessive secretion of parathyroid hormone resulting in abnormally high levels of calcium in the blood; can affect many systems of the body, especially causing bone resorption and osteoporosis) or neoplasia (the pathological process that results in the formation and growth of a tumor).

PTH concentration (parathyroid hormone) is low normal and the PTH-rP concentration (parathyroid hormone related protein) is low or undetectable in cats with Idiopathic Hypercalcemia.

Finally, radiographs may be taken of the thorax and/or abdomen to search for neoplasia. If no abnormalities are identified your veterinarian will diagnose your cat with Idiopathic Hypercalcemia (IHC).

Management of IHC

Currently, there is no known cause of Feline Idiopathic Hypercalcemia (IHC). However, experts know that feeding dry food with its inherently low water level, causes the kidneys to concentrate the urine. This concentrated urine contains minerals such as magnesium and phosphorus which may produce struvite crystals (stones) that can lead to urethral blockage in the cat. This condition is deadly.

When this condition became prevalent, pet food companies decided that the urine was too alkaline (perfect environment for struvite crystals to grow) and decided to add an acidifier and reduce the amount of magnesium in the typically dry food. The acidifiers that are commonly used are ammonium chloride or dL-methionine and some companies use phosphoric acid or sodium bisulfate. However, while struvite crystals became less prevalent in the following decade, calcium oxalate crystals became more prevalent (1990’s). These crystals grow in an environment that is too acidic.

Many experts now believe that acidifying, magnesium-restricted diets lead to IHC in genetically predisposed cats, perhaps due to the calcium resorption from bones. It is interesting to note, that carbohydrates produce an alkaline (basic) urine in the cat while meat protein produces a slightly acidic urine (pH 6.5) which is the natural pH of cat urine and has the higher magnesium level required by felines [Funaba et. al. 2003].

There is also some indication that the acidosis that occurs due to feeding acidifying diets, may also be causing kidney failure with or without hypercalcemia [Purina Nutrition Forum 1998]. Acidifying diets may also play a role in Feline Oral Resorptive Lesions (FORLs).

At least one researcher theorizes that high levels of Vitamin D may be at fault, especially in kittens fed a high Vitamin D and high calcium diet. Unfortunately, scientists do not know exactly how much Vitamin D is required by cats in their daily diet.

Loss of appetite, lethargy, mild weight loss, and constipation may be some of the signs associated with IHC. Typically, most cats with mild to moderate IHC show no signs at all.

A change in diet is warranted with the discontinuation of an acidifying diet, if one was being fed, and many experts suggest orally administered glucocorticoids or both. Diets should be wet-only to promote urinary dilution. Diets that are low-fat and high-fiber have been used with the idea that the fiber will reduce the availability of the calcium for intestinal absorption (Hill’s Prescription Diet w/d – Midkiff et al.). This diet has has been proven to be helpful with only some cats and ionized calcium will typically rise again after some time.

There is evidence that a wet-only diet of high meat protein without dL-methionine or other added acidifiers may help some of these cats. Moving toward a more natural species-appropriate diet may help the majority of the cats diagnosed with IHC, particularly the mild form. Since, as of date, there is no known mechanism to help resolve or control IHC in most of these cats. So far, research of dietary changes have not included this type of diet. Further studies are warranted. Read Dr. Mark E. Peterson’s suggestions.

Mouse – Case Study – Mouse, the Feline IHC Poster Cat  (7/17/99 – 1/2/18)  
October 2009 – Mouse was diagnosed in October 2009 with mild Idiopathic Hypercalcemia. Mouse is a neutered 10 year old male cat and has been fed a dry plus some canned diet since kittenhood. The dry food contained dL-methionine.
tCa = 12.4 mg/dL (reference range 8.9 – 10.9 mg/dL)
iCa = 1.53 mmol/L (reference range 1.00 – 1.40 mmol/L)

He was switched after diagnosis to a high-quality, canned-only, high-protein, moderate-fat, low-carbohydrate, grain-free diet (Wellness) for twelve months. No medications were administered with the diet change.
October 2010 – After twelve months of the diet change, there was a decrease in both total calcium and ionized calcium. Mouse is now 11 years old.
tCa = 9.5 mg/dL (reference range 8.2 – 11.8 mg/dL)
iCa = 1.07 mmol/L (reference range 1.03 – 1.39 mmol/L)

It has yet to be seen whether this diet, without additional medications, will prevent the calcium from rising in the future. Calcium (iCa and tCa) typically rises from months to a year or two after diet change. Whether it may be the added acidifiers, excesses of Vitamin A or D or some other dietary problem, a change to a more species-appropriate diet may help. However, what is not understood is whether the damage to the calcium receptors is a permanent condition or if given the proper diet, this may be fixed, if in fact, it is the calcium receptors that are not functioning correctly.
April 2011 – Mouse began receiving a home-prepared raw diet on December 27, 2010.
iCa = 1.14 mmol/L (reference range 1.03 – 1.39 mmol/L)
October 2011 – Mouse is still receiving the homemade raw diet. No medications have been given. Mouse is now 12 years old.
tCa = 9.0 mg/dL (reference range 8.9 – 10.9 mg/dL)
iCa = 1.30 mmol/L (reference range 1.2 – 2.0 mmol/L)
April 2012 – Mouse is still receiving the homemade raw diet. No medications have been given.
iCa = 1.23 mmol/L (reference range 1.2 – 2.0 mmol/L)
October 2012 – Mouse is still receiving the homemade raw diet. No medications have been given. Mouse is now 13 years old.
tCa = 9.2 mg/dL (reference range 8.9 – 10.9 mg/dL)
iCa = 1.40 mmol/L (reference range 1.2 – 2.0 mmol/L)
April 2013 – Mouse is still receiving the homemade raw diet. No medications have been given. In October 2012, Mouse was diagnosed with mild hyperthyroidism. He received the radioactive iodine treatment and is now euthyroid (normal thyroid function).
tCa = 9.3 mg/dL (reference range 8.9 – 10.9 mg/dL)
iCa = 0.98 mmol/L (reference range 1.2 – 2.0 mmol/L)
October 2013 – Mouse is still receiving the homemade raw diet. No medications have been given. Mouse is now 14 years old.
tCa = 8.9 mg/dL (reference range 8.9 – 10.9 mg/dL)
iCa = 1.28 mmol/L (reference range 1.2 – 2.0 mmol/L)
April 2014 – Mouse is still receiving the homemade raw diet. No medications have been given.
tCa = 9.9 mg/dL (reference range 8.9 – 10.9 mg/dL)
iCa = 1.45 mmol/L (reference range 1.2 – 2.0 mmol/L)
October 2014 – Mouse is still receiving the homemade raw diet. No medications have been given. Mouse is now 15 years old.
tCa = 10.0 mg/dL (reference range 8.9 – 10.9 mg/dL)
iCa = 1.4 mmol/L (reference range 1.2 – 2.0 mmol/L)
April 2015 – Mouse is still receiving the homemade raw diet. No medications have been given.
tCa = 9.9 mg/dL (reference range 8.9 – 10.9 mg/dL)
iCa = 1.45 mmol/L (reference range 1.2 – 2.0 mmol/L)
October 2015 – Mouse is still receiving the homemade raw diet. No medications have been given. Mouse is now 16 years old.
tCa = 9.8 mg/dL (reference range 8.9 – 10.9 mg/dL)
iCa = 1.57 mmol/L (reference range 1.2 – 2.0 mmol/L)
April 2016 – Mouse is still receiving the homemade raw diet plus a small amount of canned food. No medications have been given.
tCa = 9.6 mg/dL (reference range 8.9 – 10.9 mg/dL)
iCa = 1.05 mmol/L (reference range 1.2 – 2.0 mmol/L)
October 2016 – Mouse is still receiving the homemade raw diet plus some canned food. No medications have been given. His bloodwork is normal but he is beginning to lose weight. The lab that ran his bloodwork, made a mistake and did not run the ionized calcium test.
tCa = 10.2 mg/dL (reference range 8.9 – 10.9 mg/dL)
May 2017 – Mouse has decided he will no longer eat raw food. He is being fed Fancy Feast Classic (pate style) food. Mouse also has slightly elevated BUN and creatinine levels indicating that he has kidney disease. He will be monitored by his veterinarian for the kidney disease often. As of now, he is not receiving any medications or fluids. He is currently well hydrated. Mouse will be 18 years old on July 17.
tCa = 10.5 mg/dL (reference range 8.9 – 10.9 mg/dL)
iCa = 1.55 mmol/L (reference range 1.2 – 2.0 mmol/L)
January 2018 – Mouse started having seizures on December 27, 2017. The medication he was given made him lethargic with a loss of appetite. To spare him any pain or suffering, he was put to sleep on January 2, 2018. He was 18 and a half years old.

In lieu of a high-fiber diet, some cats that are diagnosed with IHC may be given a canned/raw only diet. The diet should consist of high-meat protein, moderate-fat, and low-carbohydrates. Magnesium levels do not need to be reduced and acidifiers should not be used. Cats may become starved for protein and fat on high-fiber diets which may also cause constipation. Suggested high-protein, grain-free canned foods are Wellness and Nature’s Variety. Nature’s Variety also produces a quality raw food. The pate versions are probably the best but check the ingredients. Other brands such as Fancy Feast Classic are affordable and palatable to most cats. The Classic versions are pate and are entirely meat-based.

Adding some raw or cooked chicken meat will also help raise the protein level of the food without increasing calcium content. However, make sure that only small amounts (less than 10% of your cat’s daily caloric intake) are fed, as the meat is not properly balanced since it does not include calcium and other nutrients derived from bone and organs. All other “treats” should be removed from the diet and fresh water should be available at all times. Feeding meal times instead of “free choice” may also help the body with control of calcium levels and urinary pH.

Remember to seek out your veterinarian’s advice before attempting any diet change. For more information on switching cats over to a canned/raw diet, please click here.

Corticosteroids (a steroid hormone produced by the adrenal cortex) promote reduction of serum calcium concentration by reducing bone resorption (reabsorption), decreasing intestinal absorption of calcium, and increasing renal excretion of calcium in the urine. Prednisone is used at 5 – 12.5 mg/day and may produce a partial or complete reduction in serum total or ionized calcium concentration during treatment. However, this increased excretion of calcium may lead to greater likelihood of calcium oxalate urolithiasis (bladder stones). A wet-only diet must be used to promote dilution of urine. Administering steroids over a prolonged period of time also has damaging effects on the body so the doctor and owner must weigh the benefits of sustained use. According to Dr. Elizabeth Hodgkins, cats that are fed a dry, high-carbohydrate food tend to have some level of liver dysfunction when on corticosteroids, however, on wet, high-meat-protein foods, this does not typically occur.

Bone Resorption Inhibitors & Alkalinizing Agents

lasix/furosemideinduces excretion
diphosphonatesinhibits bone resorption
calcitonininhibits bone resorption
mithramycininhibits osteoclastic bone resorption
sodium bicarbonatedecreases serum calcium by alkalinizing blood, thereby shifting the ionized calcium into protein bound calcium, which is less harmful
potassium citratedecreases serum calcium by alkalinizing blood, thereby shifting the ionized calcium into protein bound calcium, which is less harmful

The FVRCP combination vaccine is for Feline Viral Rhinotracheitis, Calicivirus, and Panleukopenia. Rhinotracheitis and calicivirus cause upper respiratory infections while panleukopenia (feline distemper) is deadly, most particularly to kittens. However, the cells used to grow the viruses to create the vaccines are from cat kidneys. When these kidney cells are injected into the cat (along with the vaccine), the immune system views them as foreign and makes antibodies against them, this is how vaccines work. Unfortunately, those antibodies don’t know the difference between the injected kidney cells and the cat’s own kidney tissue resulting in an autoimmune attack on the cat’s kidneys. This can lead to inflammation and permanent damage in some cats.

Prednisone is used to suppress the immune system and reduce inflammation. It is also prescribed for FIHC cats to promote reduction of serum calcium concentration. However, with the immune system already weakened due to steroid use, injection of the Rabies vaccine may further weaken the immune system and/or fail to provide immunity. Many adverse side effects can follow vaccination immediately or up to 45 days after the shot, says Jean Dodds, DVM, and can cause minor symptoms, such as swelling at the injection site, and life-threatening conditions such as seizures, and liver and kidney damage. For those living in areas that by law require the rabies vaccine, your veterinarian may provide a vaccine waiver for you to use to register your cat legally without administration of the vaccine.

Unhealthy animals should never be vaccinated. Before vaccinating an FIHC cat, discuss with your veterinarian the need, especially when the affected cat may have smaller than normal kidneys and/or kidney disease or one that is being administered steroids.

Over time, the excess calcium will deposit in the kidneys and other organs and soft tissue. The damage to the kidneys may eventually lead to renal failure. With successful management of IHC, kidney failure may be delayed indefinitely. If, or when, kidney failure does occur, there are ways to manage it as well. Please see the Feline Chronic Kidney Disease website for more information and support. You may also want to check out our page on a new way to manage cats with renal disease/failure, that is proposed by Dr. Lisa Pierson and renal disease control by Dr. Elizabeth Hodgkins.

Many cats may lead happy normal lives for many years after initial diagnosis depending on their calcium levels, management of IHC, and management of renal failure, should it occur.

In summary:

  • IHC cats should be fed a wet-only diet with no acidifiers
  • a high-quality, canned or raw, high-meat-protein, moderate-fat, low-carbohydrate, grain-free diet should be tried
  • diets comparably lower in calcium then others should be used – contact manufacturers for values
  • adding a small amount of chicken muscle meat (raw or cooked) will increase protein without increasing calcium content
  • feeding meal times instead of free choice may help the body with control of calcium levels and urinary pH
  • medications can be added if the above does not produce sufficient or lasting results
  • without successful management, IHC may lead to kidney damage and renal failure but this may also be managed for some time


The following are links and information that may help. Please seek out your veterinarian’s advice before attempting any diet change.

Cuisine-a-Crocs – a nutritionist in Belgium that will create a recipe for your cat for you to make at home.

Feeding Your Cat: Know the Basics of Feline Nutrition – Lisa A. Pierson, DVM

Feline Chronic Kidney Disease Information Center – information about renal disease in cats including management options and support for owners of afflicted cats

Feline Kidney – Waltham Center for Pet Nutrition

Feline Calcium Oxalate Bladder Stones – Wendy C. Brooks, DVM

Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice – Stephen P. DiBartola

Hypercalcemia – Wendy C. Brooks, DVM

Hypercalcemia – Canadian Vet 2009

Hypercalcemia – Heather R. Sims

Nutritional Management of Idiopathic Hypercalcemia in Cats – Dr. Mark E. Peterson

Your Cat: Simple New Secrets to a Longer Stronger Life – Elizabeth M. Hodgkins, DVM, Esq. – new information on how to feed cats with renal failure

Information on this site is for general information purposes only and is provided without warranty or guarantee of any kind. This site is not intended to replace professional advice from your own veterinarian and nothing on this site is intended as a medical diagnosis or treatment. Any questions about your animal’s health or diet should be directed to your veterinarian.

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