Concurrent Denosumab and Parenteral Iron Therapy Precipitating Severe Hypocalcemia and Hypophosphatemia

Denosumab-induced hypocalcemia and iron infusion-related hypophosphatemia are both well described. We describe a case of severe hypocalcemia and hypophosphatemia following sequential denosumab and parenteral iron administration. This resulted in respiratory failure due to muscle weakness and cardiac arrhythmia, requiring noninvasive ventilation and urgent intravenous electrolyte replacement. This case highlights the severe dysregulation in calcium and phosphate homeostasis that can occur with denosumab and iron infusions when administered in quick succession. Given that these drugs are among the most common therapies prescribed across a range of specialties, we hope to alert clinicians to this potential serious drug-drug interaction and suggest strategies for monitoring and management of the electrolyte derangement.