Hypercalcemia is a significant pathologic entity in the cat. The following summarizes an Antech Medicine Consultants’ meeting with Dr. Dennis Chew of the Ohio State University.

Causes of feline hypercalcemia include:

• Chronic renal failure
• Idiopathic hypercalcemia
• Neoplasia—lymphosarcoma, squamous cell carcinoma, myeloma and others
• Vitamin D toxicosis
• Primary hyperparathyroidism
• Granulomatous disease

Renal failure constituted 27 of 71 cases (38%) in a recent retrospective study, with 21 (30%) having neoplasia, and 4 (6%) with primary hyper-parathyroidism. In another 20 cats, the underlying pathophysiologic cause of hypercalcemia was not identified, and so could be presumed to be idiopathic.

Chronic renal failure, the most common cause of feline hypercalcemia, produces mild elevations in calcium concentration. Of the 27 cats with chronic renal failure (9 of them had urolithiasis), serum calcium concentration was 11.5 ± 0.5 mg/dL (reference range 9.2-10.2 mg/dL). Ionized calcium concentrations (Ca ++ ) are usually normal or decreased. A small % of cats with renal failure and tertiary hyperparathyroidism can have mild elevations of Ca ++ .

Idiopathic hypercalcemia produces hypercalcemia of uncertain magnitude, and with uncertain pathophysiology. Some experts believe that acidifying, magnesium-restricted diets lead to idiopathic hypercalcemia in genetically predisposed cats, perhaps due to calcium resorption from bones. Other experts claim that cats with idiopathic hypercalcemia respond to fiber-supplemented diets, although results have not been uniformly beneficial.

The cause does not seem to be occult cancer, for cats with idiopathic hypercalcemia have been monitored for as long as 3 years without developing overt neoplasia. Affected cats are usually asymptomatic, but chronic ionized hypercalcemia may lead to kidney damage and renal failure due to vasospasm, renal ischemia and necrosis, and nephrocalcinosis.

• Diagnosis—diagnosis of idiopathic hypercalcemia is made by excluding other causes. Radiographs, routine bloodwork, urinalysis, parathyroid hormone (PTH) and PTH-related protein (PTHrp) assays, and measurement of vitamin D concentrations may be necessary.
  
• Treatment—optimum treatment for idiopathic hypercalcemia is uncertain, although elevated Ca ++ concentrations should be controlled to avoid long-term renal damage. Fiber-supplemented diets may be effective in some cases. Non-acidifying, magnesium-restricted diets (e.g. renal failure diet) may also be effective. Dietary trials are typically given 2-3 months before assessing efficacy. If dietary changes fail to resolve hypercalcemia, prednisone (5mg q 12 hrs) is initiated, as this form of hypercalcemia is typically very steroid responsive with resolution within 2 weeks. Treatment is likely to be needed life-long.

Hypercalcemia of malignancy. Lymphoma, squamous cell carcinoma, and other carcinomas are the most common malignancies that can cause hypercalcemia in cats. While measurement of PTHrp concentrations may be helpful in establishing this diagnosis, the usefulness of PTH measurements in diagnosing hypercalcemia of malignancy in cats is uncertain.

Granulomatous inflammatory disease as a cause of hypercalcemia in cats is uncommon. Three of 71 hypercalcemic cats in the study mentioned above had granulomatous disease (1 with feline infectious peritonitis, 1 with cryptococcosis, and 1 with chronic nasal actiomyces rhinitis). Granulomatous injection-site vaccine reactions can also cause hypercalcemia.

Primary hyperparathyroidism is an uncommon cause of hypercalcemia in cats. Affected cats are middle aged and older, and Siamese cats may be predisposed. A cervical mass may be palpable. PTH concentrations are elevated in about 50% of cases.