PRIMARY HYPERPARATHYROIDISM WITH OSTEODYSTROPHIA FIBROSA OF THE FACIAL BONES IN A PONY

A 17-year-old pony mare was admitted for evaluation of progressive enl argement of the facial bones during the preceding 9 months. Laboratory testing revealed that the pony had hypercalcemia, hypophosphatemia, h igh urinary fractional excretion of phosphorus, and high serum concent ration of intact parathyroid hormone (185.1 pmol/L; reference range, 0 .25 to 2.0 pmol/L). On the basis of these findings, a diagnosis of pri mary hyper parathyroidism was made by ruling out nutritional secondary hyperparathyroidism, chronic renal disease, and pseudohyperparathyroi dism resulting from neoplasia. Although primary hyperparathyroidism is best treated by surgical removal of the affected parathyroid gland, t he owners declined surgical exploration of the neck in this pony. Beca use of the poor prognosis, the pony was euthanatized. A functional les ion of the parathyroid tissue was not located on necropsy or histologi c examination. Difficulty localizing and grossly identifying parathyro id tissue in horses complicates definitive diagnosis and treatment of primary hyperparathyroidism.