Severe hypophosphataemia and hypocalcaemia following intravenous ferric derisomaltose and denosumab administration

Serum calcium and phosphorus levels are tightly regulated by the calciotropic hormone parathyroid hormone, fibroblast growth factor 23 and 1,25(OH)₂ vitamin D. Commonly prescribed therapies for iron-deficiency anaemia (IDA) such as ferric carboxymaltose and ferric derisomaltose (FDM) have been shown to disrupt phosphorus homeostasis, resulting in hypophosphataemia. Similarly, denosumab use can result in hypocalcaemia due to the inhibition of osteoclastic maturation, activity and survival. Here, we report the development of severe hypophosphataemia and hypocalcaemia in a patient with osteoporosis and IDA following treatment with denosumab and FDM. The patient remained asymptomatic; however, supplementation with calcium, phosphorus and calcitriol replacement was required prior to eventual normalisation of serum levels. Often concomitantly prescribed, little guidance exists regarding electrolyte disturbances following the administration of FDM and denosumab. While hypophosphataemia and hypocalcaemia are relatively uncommon when prescribed individually, synergistic effects likely exist that warrant regular monitoring and occasional supplementation.