Severe Hypercalcemia Followed by Profound Hypocalcemia in a Patient With Familial Hypocalcemia and Metastatic Cancer

Background/Objective Severe hypercalcemia is commonly associated with malignancy and, less frequently, with excessive calcium and vitamin D intake. Management is particularly challenging in patients with underlying disorders of calcium homeostasis. We present a case highlighting the complexities of treating hypercalcemia following discontinuation of denosumab in a patient with malignancy and underlying familial hypocalcemia on long-term calcium supplementation. Case Presentation A 70-year-old woman with metastatic breast cancer and underlying familial hypocalcemia on chronic calcium and active vitamin D supplementation presented with hypercalcemic crisis following discontinuation of long-term denosumab therapy. She was treated with intravenous hydration and 2 doses of calcitonin, which led to a rapid decline in serum calcium levels and subsequent profound hypocalcemia requiring reinitiation of supplementation. Shortly after discharge, she was readmitted with recurrent hypercalcemia, again treated with 1 dose of calcitonin, followed by recurrent hypocalcemia. She was ultimately stabilized with careful titration of calcium and active vitamin D therapy. Discussion This case highlights several important clinical considerations, including the patient's familial history, oncologic history, the underlying mechanisms contributing to abrupt shifts in calcium levels, and strategies for effective management. Conclusion Treating hypercalcemia in patients with underlying hypocalcemia disorders can be challenging. Clinicians should take in consideration the complex pathophysiology of their disorder and monitor calcium closely to prevent hypocalcemia.