Internal Medicine Clinical Update: Idiopathic Hypercalcemia

Regulation of serum calcium concentration is a complex process that involves parathyroid hormone (PTH) concentration, vitamin D, and calcitonin, with PTH being the primary determinant of calcium homeostasis. The intestines, kidneys and bone are the major organs affected by PTH, with small changes in ionized calcium (iCa) producing large but opposite changes in PTH concentration; a 10% decrease in serum iCa elicits a maximal PTH secretory response.

Hypercalcemia is a well documented phenomenon with the most common causes being summarized by the acronym HARDIONS G; Hyperparathyroidism, Addison’s disease, Renal disease, Vitamin D Toxicity, Idiopathic/Iatrogenic, Osteolytic disease, Neoplasia, Spurious and Granulomatous (i.e fungal) disease.

 In cats the diagnosis of idiopathic hypercalcemia (IHC) is based on the following; Abnormally elevated serum ionized calcium concentration (iCa), low parathyroid hormone (PTH) concentration, negative parathyroid related peptide (PTH-rp), normal kidney values, normal response to ACTH stimulation testing, no evidence neoplasia (lymphoma, multiple myloma or aporcrine gland anal sac adenocarcinoma)

 The most common causes of persistent hypercalcemia in cats are idiopathic hypercalcemia (IHC) and malignancy. Cats with IHC are middle aged cats (mean 9.8 years). The majority of cats are without clinical signs and the hypercalcemia is found on wellness screening, preanesthetic evaluation or during a diagnostic evaluation for another disease. In a retrospective study of 427 cats with IHC, uncommon presenting signs included weight loss, anorexia, lethargy, polyuria and polydipsia and urinary tract signs.

 When total serum calcium is elevated, the first step is to determine if it is repeatable/persistent. The 2nd step is determine the serum iCa concentration. If the iCa is within the reference range then no specific treatment is required and serial monitoring of the calcium every 6 months would be warranted. If the iCa is elevated the following tests are recommended; CBC, biochemistry profile, PTH and PTH-related protein (PTH-rp) concentration, thoracic radiographs and an abdominal ultrasound.

In cats minor elevations in iCa are often not treated and monitored every 3 to 6 months. If the iCa is continuing to increase or there are signs of calcium toxicity (elevation in BUN, creatinine, phosphorus, decreased urine specific gravity, kidney or bladder stones or gastrointestinal signs-weight loss, constipation or vomiting) then treatment is recommended.

The first treatment recommendation is a diet change to a high-fiber veterinary diet, renal diet or calcium oxylate stone preventative diet. Currently there is some evidence for use of each of these diets but no single diet has proven more successful then the other. The iCa concentration should be reevaluated at 6 months and then every 3 to 6 months. If the iCa remains elevated then medical management is recommended with bisphosphonates and/or steroids.

My recommendations is to start with Bisphosphonates which inhibit the breakdown of bone to  decrease the serum iCa. Alendronate (Fosimax®) is an oral bisphosphonate I recommended starting at 5 to 10 mg once a week for one month. If normocalcemia is returned then serial monitoring of the iCa every 3 to 6 months is recommended. If the iCa remains elevated increasing monthly to a total of 30 mg/weekly would be recommended.

If this therapy fails to improve the iCa then the addition of 5 to 10 mg/cat of prednisolone would be recommended.. Approximate 50% of cats with IHC became normocalcemic with 5 or 10 mg/24hours.

If you have any questions with IHC or any other cases do not hesitate to contact Dr. Michael Goldstein directly at 416-784-4444 ext 1 or email at




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