A 59-year-old Female Post Bariatric Surgery with Severe Vitamin D Deficiency and Severe Hypocalcemia Induced by Denosumab Injection for Osteoporosis

Denosumab is a human monoclonal antibody that binds receptor activator of nuclear factor kappa-B ligand (RANKL) and inhibits interaction between RANKL and RANK on the surface of osteoclasts, which prevents osteoclast formation and causes decreased bone resorption and increased bone mass [1]. Osteoporosis is characterized by low bone mass, microarchitectural disruption, and increased skeletal fragility. It is common in those who have undergone Rouxen-Y gastric bypass surgery, which causes reduced gastric acid levels necessary for adequate calcium absorption and decreased vitamin D due to relative bypass of its primary absorption points in the duodenum and proximal jejunum [2,3]. Denosumab is a reasonable option for osteoporosis treatment in patients with gastric bypass surgery due to intestinal malabsorption and risk of anastomotic ulceration with oral bisphosphonates [4]. A rare yet potentially fatal adverse effect of denosumab is severe hypocalcemia and Vitamin D deficiency, which is often prolonged secondary to its long half-life and therefore difficult to treat in many cases [5]. We present a case of denosumab-induced severe hypocalcemia in a patient with history of gastric bypass who also was found to have acute severe hypovitaminosis D despite post-treatment vitamin D supplementation. Thus we hypothesize a potentially causal relationship between denosumab and acute severe vitamin D deficiency in high-risk patients, specifically with intestinal malabsorption and resultant predisposition to becoming vitamin D deficient.