Severe hypocalcemia due to osteoblastic bone metastases in a patient with hypoparathyroidism: a case report.

Objectives: Bone metastases in breast cancer are typically osteolytic but can rarely be osteoblastic. While malignancy-related hypercalcemia is well-known, malignancy-related hypocalcemia is rare and often multifactorial.

Case presentation: We present a case of a 48-year-old woman with metastatic breast cancer who developed severe hypocalcemia due to a combination of expanding osteoblastic bone metastases and hypoparathyroidism after total thyroidectomy. Despite oral and intravenous calcium supplementation, adequate calcium levels were not achieved until the patient responded to systemic therapy with Trastuzumab Deruxtecan (T-DXd), a HER2-directed antibody-drug conjugate.

Conclusion: This case underscores the challenges in finding the cause of and managing hypocalcemia in patients with complex oncological histories and emphasizes the need for close calcium monitoring in patients with bone metastases, particularly those with additional risk factors such as hypoparathyroidism or treatment with anti-resorptive drugs.