Idiopathic hypoparathyroidism was diagnosed in five young to middle-aged cats of mixed breeding. Three of the cats were male and two were female. Historic signs included lethargy (n = 5), anorexia (n = 5), muscle tremors (n = 4), weakness (n = 4), generalized seizures (n = 3), ataxia (n = 3), mental dullness or disorientation (n = 3), panting (n = 2), pruritis (n = l), ptyalism (n = 1) and dysphagia (n = 1). Weakness (n = 4), dehydration (n = 2), cataracts (n = 2), hypothermia (n = I), and bradycardia (n = I) were found on physical examination. Results of electrocardiography revealed a prolonged Q-T interval in two cats. Results of initial laboratory tests revealed profound hypocalcemia and severe hyperphosphatemia with normal renal function. The diagnosis of hypoparathyroidism was made on the basis of the history, clinical signs, and results serum biochemical testing (i-e., severe hypocalcemia and hyperphosphatemia); in two cats, the diagnosis was also confirmed by histologic examination of parathyroid glands. Initial treatment included intravenous administration of 10% calcium gluconate and oral administration of large loading doses of calcium and vitamin D (dihydrotachysterol). Successful longterm management with dihydrotachysterol and calcium was achieved in all cats. The final dosage of dihydrotachysterol required to maintain normocalcemia in the five cats ranged from 0.004 to 0.04 mg/kg/day (mean = 0.015 mg/kg/day). Long-term calcium supplementation was given to three of the cats in dosages ranging from 29 to 53 mg/kg/day (mean = 42 mg/kg/day) of elemental calcium. One cat died after 28 months of therapy from widely metastatic hemangiosarcoma; the other three cats are still HYPOPARATHYROIDISM is caused by inadequate production and secretion of parathyroid hormone (primary hypoparathyroidism) or, less commonly, by deficient end-organ responsiveness to circulating parathyroid hormone (pseudohypoparathyroidism).The major clinical manifestations of hypoparathyroidism are the result of hypocalcemia, which increases the excitability of the neuromuscular and central nervous systems. In humans and dogs, pseudohypoparathyroidism is very rare, and thus primary hypoparathyroidism accounts for most cases of the d i~o r d e r . ' ,~,~ The two most common causes of primary hypoparathyroidism are idiopathic atrophy or destruction of parathyroid tissue (idiopathic hypoparathyroidism) and iatrogenic injury or removal of the parathyroid glands during thyroid or parathyroid surgery (postoperative hypoparathyroidism).In cats, iatrogenic primary hypoparathyroidism has become relatively common due to the increased use of thyroidectomy for treatment of hyperthy~-oidism.~-' However, naturally occurring, idiopathic hypoparathyroidism (i.e., not related to neck surgery) has only been reported in one cat.* The purpose of this report is to describe the clinical signs, laboratory findings, and results of treatment in five additional cats with idiopathic hypoparathyroidism.
