Renal magnesium wasting and functional hypoparathyroidism: a diagnostic and therapeutic challenge in hypocalcemia

INTRODUCTION: Symptomatic hypocalcemia and hypomagnesemia are life-threatening emergencies. Acute diagnostic and therapeutic maneuvers are well known, but underlying etiologies and the long-term therapeutic choices are not. METHODS: A 38-year-old woman who was found to have metastatic malignant melanoma during post-iodide 131 treatment uptake and whole-body scanning for treatment of differentiated thyroid cancer presented to an emergency department after the third round of platinum-based chemotherapy with severe muscle cramping. She was found to have profound hypocalcemia that was treated. The next day, her symptoms returned, and severe hypomagnesemia was found in a visit with her oncologist. RESULTS: Blood and urine tests revealed severe renal magnesium wasting and inappropriately normal intact parathyroid hormone given her severe hypocalcemia. All electrolyte abnormalities resolved with intravenous magnesium administration over 2 days. DISCUSSION: Severe renal magnesium wasting can cause concurrent functional hypoparathyroidism. Recognition of concurrent hypomagnesemia is essential in the laboratory workup of hypocalcemia.