A 9 yr old neutered male mixed-breed dog was presented for an anal sac apocrine gland adenocarcinoma with regional nodal metastases. At presentation, ionized calcium was 1.91 mmol/L (NOVA Stat reference range, 1.1-1.3 mmol/L). Surgical excision of the primary tumor and metastatic lymph nodes was performed. Following surgery, symptomatic hypocalcemia was noted. Repeated ionized calcium measurements confirmed hypocalcemia, and hypercalcemia of malignancy panels suggested parathyroid gland suppression as the cause. The calcium normalized with parenteral calcium administration, but calcium later became elevated with tumor recurrence and an increase in the parathormone-related peptide. Disrupted calcium homeostasis is a potential complication following the treatment of long-standing humoral hypercalcemia of malignancy.