Hypercalcemia due to Co-Occurring Metastatic Breast Cancer and Primary Hyperparathyroidism

Hyperparathyroidism and malignancy are both causes of hypercalcemia. Breast cancer patients usually have hypercalcemia due to metastases or paraneoplastic syndrome resulting from ectopic PTHrP production. Occasionally, other causes such as primary hyperparathyroidism may co-exist and contribute to the hypercalcemia as well. We present the case of a 61 year old with a history of breast cancer with bony metastasis who presented with a corrected calcium level of 17.9 mg/dl. Bloodwork and imaging was suggestive of primary hyperparathyroidism. This case highlights the rare co-existence of dual etiologies of hypercalcemia and provides an overview of the presentation, diagnostic approach and management in such scenarios.