Primary hyperparathyroidism caused by a functional parathyroid adenoma in a horse.

A 14-year-old Arabian gelding had weight loss and anorexia of 3 weeks' duration. Results of repeated laboratory tests revealed persistent hypercalcemia and serum phosphorus concentration that was within or less than the reference range. Parathyroid hormone concentration was high. Histologic examination of specimens obtained at necropsy revealed parathyroid adenoma. A diagnosis of primary hyperparathyroidism attributable to a functional parathyroid adenoma was made. Abnormalities in calcium and phosphorus concentrations were similar to those seen with primary hyperparathyroidism in dogs, in which this syndrome is best described. Primary hyperparathyroidism should be considered to be a potential cause of hypercalcemia in horses in which other more common causes of hypercalcemia, such as humoral hypercalcemia of malignancy, nutritional secondary hyperparathyroidism, chronic renal failure, vitamin D toxicosis, and bony or granulomatous disease, are ruled out.