A 13-year-old spayed Labrador Retriever cross dog presented for polyuria and polydipsia. Serum total calcium, free calcium, and intact parathyroid hormone concentrations were elevated. Surgical exploration of the ventral neck revealed a grossly enlarged right external parathyroid gland. The histopathological diagnosis for the excised right parathyroid gland was an incompletely resected parathyroid carcinoma. Parathyroid carcinoma in the dog is an infrequent cause of hypercalcemia and primary hyperparathyroidism.Keywords canine, hypercalcemia, histopathology, parathyroid glands A 13-year-old spayed Labrador Retriever cross dog (26 kg) was referred with a 6-week history of polyuria and polydipsia.A complete blood cell count (CBC), serum biochemistry panel, and urinalysis (cystocentesis sample) were evaluated. The CBC revealed a mild lymphopenia (0.966 Â 10 9 /liter; reference interval, 1.2-5.0 Â 10 9 /liter), which was attributed to stress. Abnormalities in the serum biochemistry panel included a mildly elevated serum cholesterol concentration (7.28 mM; reference interval, 2.70-5.94 mM), increased serum alkaline phosphatase (ALP) activity (176 U/L; reference interval, 9-90 U/L), and mildly increased total calcium concentration (3.84mM; reference interval, 1.91-3.03mM). The free calcium concentration was elevated (1.87mM; reference interval, 1.25-1.45mM). Further testing revealed an increased intact parathyroid hormone (iPTH) concentration (38.6pM; reference interval, 3-17pM). The high cholesterol was most likely a postprandial elevation because a nonfasted sample was collected. An increase in the steroidinduced and/or hepatic isoforms of ALP could explain the elevated serum ALP activity. The urine was isosthenuric (urinespecific gravity, 1.008). Ancillary diagnostics included plain film thoracic and abdominal radiographs, which were unremarkable. Based on the presence of elevated free calcium and iPTH and the lack of detection of another neoplasm, primary hyperparathyroidism was considered to be the most likely diagnosis. The dog was taken for surgical excision of the external parathyroid glands, which were submitted for histopathological examination. Differential DiagnosisHypercalcemia in the dog can have many etiologies, including malignancy (eg, primary hyperparathyroidism, humoral hypercalcemia of malignancy, multiple myeloma, and neoplasia in bone), renal failure, hypoadrenocorticism, and increased vitamin D activity (eg, rodenticides containing cholecalciferol, compounds containing calcipotriene or calcipotriol, plants containing calcitriol glycosides, excess dietary supplementation, and granulomatous inflammation).15 Hypercalcemia of malignancy is the most common cause of hypercalcemia in the dog 3,15 and is most often due to lymphosarcoma or apocrine gland adenocarcinoma of the anal sac.15 It is less commonly present with other malignancies, including multiple myeloma and carcinomas. 15 In primary hyperparathyroidism, the most common tumors of the parathyroid glands are adenomas, 2 whereas hyperpla...