Why Is My Dog Or Cat’s Blood Calcium Level High Or Low?

Ron Hines DVM PhD

To see what normal blood and urine values are for your pet, go here

For an explanation of causes of most abnormal blood and urine tests go here

To see how tests are often grouped, go here

Your pet’s free, ionized or available blood calcium can often tell your veterinarian more. Read about that test here 

The Calcium Level Of Your Dog And Cat’s Blood

Although ~99% of the calcium in your cat or dog’s body is stored in its bones and teeth for skeletal and dental strength, the 1% that is bound in those two areas has critical functions in your pet’s body other than for its strength and rigidity. Your pet’s bones serve as the storage bank for that important 1% of free circulating calcium. It is critical for many, day-to-day, life processes that occur within your pet’s body. Much of that “free” calcium is actually bound to blood albumin protein but another portion, the active portion, is free to support vital functions.

Here are some of the important roles of free calcium in your pet’s body:

Circulating calcium plays an important role in the action of many basic enzymes (enzymatic processes). For example, proteases enzymes that cells require to disassemble proteins into their component amino acids require calcium. (ref)   These same enzymes are a necessary part of your pet’s immune system as well. (ref)

Sufficient blood calcium must be present before your pet’s blood can clot. (ref)

Circulating calcium also plays a part in keeping your pet’s body fluids in their proper place (by contributing to osmotic pressure). (ref)

Calcium is essential for normal muscle contractions. (ref)

When your dog or cat ’s dietary intake of calcium is low, your pet will maintain its blood calcium levels within the proper range even at the expense of its bone strength — for as long as it possibly can.

A general (entire body) calcium deficiency in your dog or cat is not always due to insufficient calcium in its diet. It can also be because of chronic intestinal or kidney disease, (lack of activated vitamin D3 (ref) that prevents proper calcium absorption (=Secondary hyperparathyroidism) or because the ratio of calcium to phosphorus in your pet’s diet is not optimal (it should be about 1:1 to 2:1). In unsupplemented meat diets that ratio is reversed.

What Symptoms Might I See If My Dog Or Cat’s Blood Calcium Level is Lower Or Higher Than Normal?

None of the symptoms you might observe are specific or diagnostic in themselves. In mild cases of low or high blood calcium you probably won’t see any symptoms in your pet. When the problem is lower-than-normal blood calcium (hypocalcemia) you probably would not notice that a problem existed until total blood calcium level in your dog or cat fell below 6.7 mg/dL.

Reasons Why Your Pet’s Blood Calcium Level Might Be High (hypercalcemia):

Veterinarians see more cases of abnormally high blood calcium levels than low blood calcium levels (about 60% of those with abnormal calcium numbers are above normal).

Calcium values can be slightly higher than suggested normal values in immature and growing dogs and cats (particularly the puppies of giant dog breeds).

High Blood Albumin Protein Levels As A Cause of Abnormally High Blood Calcium Level

Higher than normal blood albumin levels have the ability to cause higher than normal blood calcium levels. The most common cause for a higher than normal blood albumin level in pets is dehydration (hemoconcentration, due to lack of water intake, diarrhea or vomiting ). Read more about abnormal blood albumin readings here.

Cancer

When your pet’s blood albumin level is normal, the most common cause for abnormally high blood calcium level is a tumor (cancer, malignancy) (due to a paraneoplastic syndrome =PNS [ ref] ). These are indirect effects of certain types of tumors. The most common ones in dogs are  lymphosarcomas and anal sac adenocarcinomas. In cats, lymphomas and squamous cell and bronchogenic carcinomas are the tumors most often responsible. Multiple myelomas (immune system tumors) , thymomas (thymus gland tumors) , osteosarcomas (bone tumors) , and fibrosarcomas also have the potential to raise your pet’s blood calcium level.

Chronic Kidney Disease (Kidney disease can cause blood Calcium to go up; but generally causes it to go down)

Chronic kidney disease often cause blood calcium levels in dogs and cats to be abnormally low. But kidney issues in your pet will occasionally cause blood calcium levels to go up  – (too high). More pets with kidney issues have calcium levels go down (hypocalcemia)  than up (hypercalcemia) . It depends a lot on what portion of the blood calcium your vet is measuring and at what point in the disease process the blood sample was taken. When in doubt, measure the free ionized calcium portion – not total blood calcium.

The calcium in your pet’s blood exists in three forms: part is free ionized calcium , part is bound to blood protein and part is in “complex” with ions (anions) in your pet’s blood. In dogs and cats, one can be abnormal while the others remain normal. In cats with chronic kidney disease, about 6% have abnormally high ionized blood calcium while 26% have abnormally low ionized blood calcium levels. By the time these cats are experiencing advanced kidney disease, more than half will have low blood ionized calcium levels despite one in five having high total blood calcium levels (yes, I know it is confusing).

Dogs probably experience the same abnormalities in their chronic kidney disease (CKD). When you know that your pet has kidney disease, measuring its free ionized calcium level is the better test to choose (but more expensive and harder to perform).

High blood calcium can be both the cause and the result of kidney disease. Certain rat and mouse poisons contain vitamin D that can damage your pet’s kidneys through excessive calcification (metastatic calcification). Your pet’s eating Day Blooming Jasmine can have the same effect. High blood calcium levels have been reported to cause increased urination, perhaps through its ability to interfere with the action of antidiuretic hormone (ADH,AVP, vasopressin) released by your pet’s pituitary gland.

Sudden or Acute Kidney Disease can also be responsible for high blood calcium levels. That can be caused by urinary tract blockage (oxalate or struvite stones, cancer etc.) (ref1, ref2), diseases like leptospirosis, and even grape toxicity). High blood calcium levels can also be found in dogs that consume the artificial sweetener Xylitol (ref)

Rarer Causes of High Blood Calcium In Dogs And Cats:

Certain bone infections (osteomyelitis) can liberate calcium, leading to elevated blood calcium levels. Addison’s disease in dogs or acromegaly  in cats can also be responsible for high blood calcium levels. High blood calcium levels are also occasionally seen in pets that are infected with some rare fungal diseases (blastomycosis, histoplasmosis, and coccidioidomycosis) and other infections that cause granulomas.

Primary Hyperparathyroidism and Hypercalcemia in Dogs (PHPTH)

Your pets multiple parathyroid glands, located in its neck (see this illustration), are responsible for regulating the amount of calcium in its blood. Occasionally, one or more of these gland produce its hormone, PTH, in excessive amounts. When seen, keeshonds, labs, German shepherds and cockers top the list of affected breeds. About 40% do not appear ill. In the rest, excessive thirst and urination as well as decreased appetite and activity, shivering and twitching are the most common symptoms. There are theories, but we really do not know the underlying cause of PHPTH. 

Primary Hyperparathyroidism in Cats

Cats develop PHPTH too – but even less commonly than dogs. It is even less common in cats than in dogs to ever determine the underlying cause. Some of these cats go on to be diagnosed with cancers (leukemias, osteosarcomas, fibrosarcomas, carcinomas) others with kidney failure the probable underlying cause.

But in most cats, the underlying cause can never be determined. That is why it is often called Feline “Idiopathic” Hypercalcemia. That is a round-about way of saying “we know what’s wrong but we don’t know why”.  Cats with this problem tend to be older (ave age 9 yrs). The most common signs are weight loss (due to anorexia = no appetite) and inactivity (lethargy). Some cats with this problem also vomit or have diarrhea/constipation issues. Many of these cats have a calcium oxalate urinary tract problem as well. Some show abnormal calcification of their kidneys (neprhrocalcinosis). When looking for this condition, measuring your pet’s ionized calcium level is more likely to pick up the problem than a total blood calcium level assay. Idiopathic Hypercalcemia is not the most common cause of high blood calcium levels in cats. Often, it is picked up on a routine health screen without the owners describing a specific health issue in the pet. In one study, only 46% of the cats with high blood calcium had signs of ill health. Some of the cats had mild weight loss, others had IBD, and a few (5% or less) had constipation issues, vomiting or a drop in appetite. Interestingly, 10-15% had urinary tract stones. 

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Occasional cats seem to benefit from a diet containing lower-than-normal calcium content (about 40% of NRC feline recommendations for calcium) and restricted amounts of vitamin D3. When the problem is suspected, ionized calcium is the portion of blood calcium most likely to show the elevation. It is possible that a diet low in calcium but high in phosphorus may be a contributing cause. If you home cook your pet’s food , keep that in mind.

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High blood calcium levels have been reported in pets that accidentally ate their owners anti-psoriasis medications. (ref) Or that consumed excessive amounts of calcium-containing or vitamin A-containing products. The consumption of excessive vitamin D3, either as a vitamin supplement or in rodent poison (Vit D3 = cholecalciferol) can also cause abnormally high blood calcium readings in pets. Lipemic (fat =lipid containing blood – usually due to not-fasting your dog or cat before the sample was taken)  can give falsely high calcium readings. So can accidentally leaving detergent in the blood collection tube or equipment.

Reasons Your Dog Or Cat’s Blood Calcium Level Might Be Low (hypocalcemia):

Just as in situations where your pet’s blood calcium levels are abnormally high, situations where they are abnormally low don’t present any unique signs. But many dogs and cats with severe hypocalcemia have muscle tremors and twitching. They might walk stiffly, due to cramping, or appear anxious and preoccupied. Others pant, run fevers, (as in eclampsia) or have abnormally fast heart rates. Those with total blood calcium levels below 4.5 mg/dL are at risk of dying. 

Low Blood Albumin Levels (Hypoalbuminemia)

Abnormally low blood albumin protein levels (hypoalbuminemia) is the most common reason a laboratory report from your pet comes back reporting a low blood calcium level (probably about half of the lab test reports that report low blood calcium levels are low in albumin as well). Generally the calcium level is not low enough for your dog or cat to show low-calcium symptoms. Your pet’s blood albumin binds to and carries much of the calcium in its blood stream. When insufficient albumen is present, the total amount of calcium in circulation decreases. In these situations, the pet’s non-bound or free ionized blood calcium generally remains normal. Dogs and cats with chronic diarrhea due to inflammatory bowel disease sometimes develop low blood albumen protein level due to their blood albumin escaping through their damaged intestinal lining (protein loosing enteropathy). Read more about IBD  in dogs here,  and in cats here. Without enough circulating albumen protein your pet’s, total blood calcium level will drop. 

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The two next most common causes of serious hypocalcemia (low blood calcium level) are long term (chronic) or sudden (acute) kidney failure and eclampsia (milk fever).

Kidney Failure (remember, it appears in both the up and the down causes for blood calcium)

A major job of your dog and cat’s kidneys is to keep the amount of dissolved blood ingredients at their proper levels. When levels of many ingredient are too high, the kidneys remove the required amount and deposit it in the pet’s urine for elimination. When levels are too low, that ingredient is conserved. When your pet’s kidneys are failing, they loose that ability. One blood ingredient that tends to increase dramatically in those situations is phosphorus (blood phosphate).  In cases of high blood phosphorus, blood calcium levels inevitably fall. The reason for this is unclear – some suspect that the lost calcium has combined with the phosphorus and been deposited in the pet’s bones as a form of calcium phosphate (hydroxyapatite). Both long term kidney decline (CRF) or sudden kidney damage can have this effect ( produce hypocalcemia). The drop in blood calcium might be more pronounced when your pet’s kidney damage occurred suddenly and its body just didn’t have enough time to adjust. When the kidney damage is long term (chronic), a condition is called renal secondary hyperparathyroidism is common. It’s seen mostly in dogs, but can occur in cats as well. The high phosphorus levels produced by kidney damage , cause the dog and cat’s parathyroid glands to secrete excess PTH hormone in an attempt to move calcium from the pet’s bones into its blood stream. These damaged kidneys also fail to produce sufficient active-form vitamin D(D3 aka calcitriol) , which is required to absorb adequate new calcium from foods. The end results for the pet are weakened bones that easily fracture.  In some dogs, the lower jaw (mandible) becomes abnormally flexible as the calcium that gives it its strength is slowly lost (“rubber jaw”).  Many cats with hyperparathyroidism suffer from it as the result of kidney disease and hyperthyroidism. But in a few cats, their total blood calcium will actually be higher than normal – probably because their lack of kidney-produced vitamin D3 required to absorb calcium from their food outweighs their increased production of parathyroid hormone PTH) which has the opposite effect on blood calcium levels. I know that this is quite confusing. 

Eclampsia 

Sometimes mother pets have problems mobilizing their body’s calcium reserves near the time that they give birth. The problem is  called eclampsia. It is also called pregnancy-related milk fever, parturient paresis or puerperal tetany. Eclampsia is most common in toy dog breeds, mothers of first time litters, or pets that have been over-supplemented with calcium during pregnancy. The problem is much rarer in cats. The most common signs of eclampsia are muscle tremors and twitching, weakness, agitation, leg stiffness and a reluctance to stand or move. Occasionally, seizures also occur. Despite the fact that this problem responds well to giving these pets intravenous calcium along with medications to help lessen their muscle tenseness, total blood calcium levels in these pets are more likely to be within the normal range when measured than they are to be low. Measuring your pet’s ionized calcium is a more reliable way to confirm the problem is eclampsia. (ref)

Less Common Causes Of Low Blood Calcium Levels:

Phosphate Enemas

Many human enema preparations (eg Fleet™ enemas) contain phosphate, and, as I mentioned before, anything that makes your pet’s blood phosphate level rise above normal will make its blood calcium level drop. That is why most veterinarians rely on  soapy warm water enemas instead. The soap ingredient is always a very mild vegetable oil based soap.

Pancreatitis

 Acute pancreatic inflammation in dogs and cats often causes a mild-to-moderate drop in blood calcium levels. Veterinarians are not certain why that is, but some think that the leakage of a pancreatic enzyme (lipase) into surrounding tissue plays a part in tying up circulating calcium. A pancreatic hormone called glucagon, that might be released in large amounts during an attack of acute pancreatitis, might also play a role (by stimulating calcitonin release) but  if your pet vomits severely enough and becomes dehydrated, its blood calcium level could actually rise. ( ref )

Hypoparathyroidism = Sluggish, Inactive or Missing Parathyroid Glands

The responsibility of your pet’s multiple parathyroid glands that are located in its neck region to keep the dog or cat’s blood calcium levels adequate. The glands do that by constantly monitoring the pet’s blood calcium level and releasing PTH hormone when calcium levels drop too low. It is quite rare for the parathyroid glands to naturally loose their ability to perform that task.  So “natural” hypoparathyroidism is quite rare in pets. Seventeen dogs came into the University of Murdoch Veterinary Clinic with this problem between 1990-2004. Seizures, muscle tremors and twitching, stiff gait, rigidity, muscle cramping, behavioral change and hyperventilation were the most common symptoms. All dogs had abnormally low blood calcium levels and mildly elevated blood albumin levels. (ref

Much more common than naturally occurring  hypoparathyroidism is accidental (iatrogenic) removal of the parathyroid glands during thyroid gland or neck surgery or due to a neck injury or nearby pathology (tumors etc.) that destroys the parathyroid glands.

Diets That Are Deficient in Calcium and/or Vitamin D3

Poorly thought out diets, such as those that are composed primarily of unsupplemented red meat or fish fillets are all high in phosphorus and low in calcium. (read about that  here) Young dogs and cats, unfortunate enough to be fed these diets develop rickets. Older ones develop weak bones (osteomalacia). The pet’s parathyroid glands are aware of the situation and attempt to correct it by releasing more PTH hormone to free more calcium from the bones . This condition is called  nutritional secondary hyperparathyroidism. Diets that are deficient in vitamin D3 – as all-red meat diets are (ref) – will not allow proper calcium absorption (hypovitaminosis D). Pets fed diets that are deficient in magnesium can also develop hypocalcemia because sufficient blood magnesium content is required for proper parathyroid gland function. Intestinal malabsorption due to IBD in your dog or in your cat as part of Triad disease and similar digestive tract issues can prevent your pet from absorbing sufficient calcium from its diet. The same effect can occur when insufficient food is available (starvation) or when your pet is reluctant or unwilling to eat for any reason. 

Rhabdomyolysis and Soft Tissue Trauma

Rhabdomyolysis is a complicated word that essentially means “muscle damage”. When such a situation occurs (such as afterextreme exertion or subsequent to muscle trauma), injured muscle cells release a lot of phosphate/phosphorus. I mentioned that as blood phosphate goes up, blood calcium goes down. That, plus the kidney damage that often accompanies severe muscle damage, is thought to be the reason your pet’s blood calcium levels fall. The muscle damage associated with the bite of a venomous snake has also been associated with low total blood calcium. 

Antifreeze Poisoning And Other Toxic Compounds

Ethylene glycol poisoning causes large amounts of oxalate to be produced in your pet’s body. That oxalate binds to circulating blood calcium to form calcium oxalate. So blood calcium levels fall. Ethylene glycol crystals also damage your pet’s kidneys’ ability to produce vitamin D3 and to retain blood albumin – both of which are required to keep its blood calcium at normal levels.  Antifreeze is not the only product that can cause oxalate poisoning leading to hypocalcemia. Certain plants (including lilies and, philodendron) can contain large amounts of oxalate. (ref)

Certain Cancers

Medullary carcinoma of the thyroid gland and primary (when it began there) and metastatic (when the cancer moved there) bone tumors have also been associated with hypocalcemia.

Anticonvulsant/Epilepsy  Therapy

Phenobarbital or phenytoin (although neither are frequently given to dogs or cats with epilepsy) are known to lower blood vitamin D levels. That could lead to lower blood calcium levels. (ref)

Blood Collection And Other Lab And Clinical Errors

Accidental use of blood collection tubes that contained EDTA, citrate or oxalate anticoagulants will give falsely-low blood calcium readings. Pets that receive substantial blood transfusions containing citrate anticoagulant can also have lower blood calcium readings because the citrate ties up their blood calcium.  Pets that in emergencies receive large amounts of IV fluids that have no calcium added to them can simply dilute the amount of calcium in the pet’s blood stream. Pets that receive intravenous phosphate to correct abnormally high blood calcium levels sometimes subsequently dip too low in their blood calcium. Therapeutic doses of sodium bicarbonate (NaHCO3), used to correct metabolic acidosis can have the same effect (ref)

Blood drawn from pets that are lipemic either because of a recent meal or because of metabolic problems such as hypothyroidism are sometimes said to show falsely elevated blood calcium levels (the cloudy blood serum sample interferes with the accuracy of some blood analysis machines).

Medications That Have Been Associated With Low Blood Calcium

I mentioned that sodium bicarbonate that is sometimes given intravenously or orally to your pet to correct acidic situation  can lower blood calcium level and that certain human anti-seizure medications can too. But both enrofloxacin (Baytril®) and tetracycline antibiotics have also been associated with low blood calcium readings. That same effect has been seen when administering furosemide (Lasix®) diuretic.

Complimentary Tests:

CBC and blood chemistry panel, more specifically, run blood phosphorus, glucose, BUN, Creatinine [kidney function], potassium, urinalysis, ionized calciumEKG , PTH levels for parathyroid gland function, Parathyroid hormone related protein (PTHrP) = test (for evidence of cancer), Review of diet and nutrition, Note if hypocalcemic tetany improves with IV calcium. Body temperature for fever, amylase, and pancreas-specific lipase (when pancreatitis is suspected) T4 for hyperthyroidism or hypothyroidism, , ACTH stimulation test (to access adrenal gland function wen required), [This:  diagnostic flowchart for low blood calcium might be helpful to you as well]

DxMe

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