Persistent and recurrent hypercalcemia are uncommon in dogs undergoing surgical treatment for primary hyperparathyroidism

Objective: To report the proportion of cases, causes, and definitions of recurrent and persistent hypercalcemia in dogs following surgical removal of autonomously functioning parathyroid tissue. Methods: Retrospective review of medical records from 3 academic veterinary hospitals (2012 to 2022) identified dogs with surgically treated primary hyperparathyroidism. Data included signalment, clinical signs, diagnostics, treatment, histopathology, and outcomes. Dogs were categorized by follow-up: ≤ 6 months (group 1) or > 6 months (group 2). Descriptive statistics were performed. The Mann-Whitney U test compared preoperative ionized calcium and parathyroid hormone between dogs with persistent versus resolved hypercalcemia. Results: 202 dogs met the inclusion criteria. Group 1 included 123 dogs, and group 2 included 79 dogs. Mean age was 10.7 ± 2.23 years, with no sex predilection. Hypercalcemia-associated clinical signs occurred in 68% (138 of 202) of dogs. Histopathology revealed parathyroid adenoma (58% 135 of 233), hyperplasia (24% 56 of 233), and carcinoma (7% 10 of 233) as most common. Persistent hypercalcemia occurred in 25 of 202 dogs (12.4%), primarily due to incorrect surgical removal or multiglandular disease. Preoperative ionized calcium and parathyroid hormone did not differ between persistent and resolved cases. Recurrent hypercalcemia occurred in 1 of 79 (1.3%) dogs in group 2, associated with a new mass. Fifteen dogs underwent a second surgery; 1 dog required a third surgery. Conclusions: Persistent and recurrent hypercalcemia were uncommon (12.4% and 1.3%, respectively). Multiglandular disease and incorrect surgical excision were the leading causes of persistent hypercalcemia. Clinical Relevance: This study emphasizes that persistent and recurrent hypercalcemia are uncommon but require diligent perioperative planning and monitoring.