Denosumab-induced hypocalcemia post bariatric surgery—a severe and protracted course: a case report

Background Denosumab is known to cause abnormalities in calcium homeostasis. Most of such cases have been described in patients with underlying chronic kidney disease or severe vitamin D deficiency. Previous bariatric surgery could also contribute to hypocalcemia and deterioration in bone health. Case presentation We present a case of a 61-year-old Malay female with worsening bilateral limb weakness, paresthesia, and severe carpopedal spasm a week after receiving subcutaneous denosumab for osteoporosis. She had a history of gastric bypass surgery 20 years ago. Post gastric bypass surgery, she was advised and initiated on lifelong calcium, vitamin D, and iron supplementations that she unfortunately stopped taking 5 years after surgery. Her last serum blood tests, prior to initiation on denosumab, were conducted in a different center, and she was told that she had a low calcium level; hence, she was advised to restart her vitamin and mineral supplements. Laboratory workup revealed severe hypocalcemia (adjusted serum calcium of 1.33 mmol/L) and mild hypophosphatemia (0.65 mmol/L), with normal magnesium and renal function. Electrocardiogram showed a prolonged QTc interval. She required four bolus courses of intravenous calcium gluconate, and three courses of continuous infusions due to retractable severe hypocalcemia (total of 29 vials of 10 mL of 10% calcium gluconate intravenously). In view of her low vitamin D level of 33 nmol/L, she was initiated on a loading dose of cholecalciferol of 50,000 IU per week for 8 weeks. However, despite a loading dose of cholecalciferol, multiple bolus courses, and infusions of calcium gluconate, her serum calcium hovered around only 1.8 mmol/L. After 8 days of continuous intravenous infusions of calcium gluconate, high doses of calcitriol 1.5 μg twice daily, and 1 g calcium carbonate three times daily, her serum calcium stabilized at approximately 2.0 mmol/L. She remained on these high doses for over 2 months, before they were gradually titrated down to ensure sustainability of a safe calcium level. Conclusion This case report highlights the importance of screening for risk factors for iatrogenic hypocalcemia and ensuring normal levels before initiating denosumab. The patient history of bariatric surgery could have worsened the hypocalcemia, resulting in a more severe presentation and protracted response to oral calcium and vitamin D supplementation.