High calcium in cats, known as hypercalcemia, is most commonly caused by one of three conditions: idiopathic hypercalcemia (meaning no identifiable cause), cancer, or chronic kidney disease. These three account for the majority of cases, though vitamin D toxicity, overactive parathyroid glands, and other less common conditions can also drive calcium levels up. The normal range for ionized calcium (the biologically active form) in cats is 1.25 to 1.45 mmol/L, and values above that threshold signal a problem that needs investigation.
Idiopathic Hypercalcemia: The Most Frustrating Diagnosis
Idiopathic hypercalcemia is one of the top causes of elevated calcium in cats, and it’s also the least understood. “Idiopathic” simply means the cause is unknown. Your vet has ruled out cancer, kidney disease, and every other identifiable trigger, and the calcium is still high. Some analyses rank it as the single most common cause of feline hypercalcemia, while a large database review from Cornell University (covering 2007 to 2017) found that malignancy and kidney failure were actually more frequent. Either way, it’s a diagnosis vets encounter regularly.
The suspected mechanisms behind idiopathic hypercalcemia involve a disruption in how the body handles calcium. The problem could stem from too much calcium being pulled from bones, too much being absorbed from food in the intestines, too little being filtered out by the kidneys, or some combination of all three. Chronic, low-grade inflammation may also play a role: certain inflammatory signaling molecules activate the cells that break down bone while suppressing the cells that build it, releasing stored calcium into the bloodstream.
Many cats with idiopathic hypercalcemia show no obvious symptoms at all. The condition is often discovered incidentally during routine bloodwork. When signs do appear, they tend to be vague: reduced appetite, weight loss, or increased thirst and urination. This makes it easy to miss without regular lab testing.
Cancer and Calcium
Cancer is a well-established cause of high calcium in cats. Lymphoma is the tumor type most commonly associated with hypercalcemia in felines, though squamous cell carcinoma and other malignancies can also be responsible. Tumors raise calcium levels in a few different ways. Some produce a protein that mimics parathyroid hormone, tricking the body into releasing calcium from bones and retaining it in the kidneys. Others directly invade bone tissue, breaking it down and flooding the bloodstream with calcium.
Hypercalcemia caused by cancer tends to produce more severe and noticeable symptoms than the idiopathic form. Cats may lose weight rapidly, stop eating, vomit, or become profoundly lethargic. If your vet finds high calcium alongside other worrying lab results or physical exam findings, cancer screening (including imaging and sometimes biopsies) will be a priority. Treating the underlying tumor is the most effective way to bring calcium back down in these cases.
Chronic Kidney Disease
Cats with chronic kidney disease have an increased risk of developing high calcium levels, and the relationship between the two conditions runs in both directions. As kidney function declines, the kidneys lose the ability to properly filter phosphorus out of the blood. That phosphorus buildup sets off a chain reaction involving several hormones, including parathyroid hormone and a phosphorus-regulating hormone called FGF23, that ultimately disrupts calcium balance.
Interestingly, one of the standard treatments for kidney disease, restricting phosphorus in the diet, may itself contribute to hypercalcemia in some cats. When phosphorus drops too low, the hormonal signals that normally keep calcium in check can overshoot. Cats with kidney disease who are also dehydrated tend to develop hypercalcemia faster and become sicker, because the kidneys can’t excrete excess calcium efficiently when they’re not getting enough fluid.
This two-way relationship means that high calcium can also damage the kidneys. Persistent hypercalcemia causes mineral deposits to form in kidney tissue, accelerating the decline of an organ that may already be compromised. Managing both conditions simultaneously requires careful monitoring of bloodwork and diet.
Vitamin D Toxicity
Excessive vitamin D intake is a less common but serious cause of high calcium in cats. Vitamin D is converted in the liver and then activated in the kidneys, and in its active form it dramatically increases calcium absorption from the gut. When a cat takes in too much, calcium levels can spike to dangerous levels within 12 to 72 hours, sometimes reaching as high as 20 mg/dL alongside a rise in phosphorus.
There are two main ways cats get exposed to toxic amounts of vitamin D. The first is accidental ingestion of rodenticides that contain a form of vitamin D (cholecalciferol). These products kill rodents by causing fatal hypercalcemia, and they’re equally dangerous to cats. The second, more insidious route is through pet food manufacturing errors. Documented case series have linked vitamin D toxicity in cats to commercial diets that were accidentally over-supplemented with vitamin D3 during production. Along with high calcium and phosphorus, affected cats typically show rising kidney values on bloodwork and can develop mineralization of soft tissues like the kidneys and blood vessels.
Overactive Parathyroid Glands
The parathyroid glands are tiny structures near the thyroid that regulate calcium levels by producing parathyroid hormone (PTH). When one or more of these glands develops a benign tumor (adenoma) or simply becomes overactive, it pumps out too much PTH, which pulls calcium from bones and tells the kidneys to hold onto it instead of filtering it out. This condition, called primary hyperparathyroidism, is less common in cats than in dogs but does occur. It’s typically diagnosed by finding elevated calcium alongside an inappropriately high PTH level on bloodwork.
Other Less Common Causes
Several additional conditions can raise calcium in cats, though they’re encountered less frequently:
- Granulomatous disease: Fungal infections and certain inflammatory conditions can produce activated vitamin D outside the kidneys, increasing calcium absorption.
- Addison’s disease: Adrenal gland failure (rare in cats) can cause mild hypercalcemia due to dehydration and changes in how the kidneys handle calcium.
- Bone disease: Conditions that cause excessive bone breakdown can release stored calcium into circulation.
Signs to Watch For
High calcium primarily affects three body systems: the urinary tract, the digestive system, and the nervous system. The most common early signs are increased thirst and urination, because the kidneys work harder to flush excess calcium. Over time, that calcium can form bladder or kidney stones and increase the risk of urinary tract infections.
Digestive symptoms include loss of appetite, nausea, vomiting, and constipation. Cats with persistently high calcium often lose weight. On the neurological side, you may notice general weakness, lethargy, muscle trembling, or difficulty getting up. In some cases, high calcium also affects the cardiovascular system, leading to high blood pressure or abnormal heart rhythms.
The tricky part is that many cats, particularly those with idiopathic hypercalcemia, show no symptoms at all until levels climb high enough to cause real damage. This is one reason routine bloodwork is valuable, especially in middle-aged and older cats.
How Vets Identify the Cause
Finding high calcium on a blood panel is just the starting point. The real challenge is figuring out why it’s elevated, because treatment depends entirely on the underlying cause. Your vet will typically measure ionized calcium (not just total calcium) for accuracy, then run a series of tests to narrow down the source.
Key tests include parathyroid hormone levels (to check for overactive parathyroid glands), a related protein called PTHrP (which tumors often produce), and vitamin D metabolite levels (to rule out toxicity or granulomatous disease). A full chemistry panel assessing kidney function and phosphorus is standard. Imaging such as X-rays, ultrasound, or advanced imaging may follow to look for tumors or kidney stones. If all identifiable causes are ruled out, the diagnosis defaults to idiopathic hypercalcemia.
Because several of these causes look similar on basic bloodwork, the diagnostic process can take multiple rounds of testing. Patience with this workup matters, since the treatment for cancer-driven hypercalcemia is fundamentally different from managing a dietary cause or an idiopathic case.