Medullary thyroid cancer (MTC) is a rare subtype of thyroid cancer originating from parafollicular C-cells of the thyroid. Tyrosine kinase inhibitors are used to treat patients with advanced MTC. Selpercatinib is a highly selective RET inhibitor used in the treatment of advanced RET-mutated MTC, having shown higher potency and fewer side effects compared to multikinase inhibitors in clinical trials. As a relatively new drug, its toxicity profile continues to be characterised. This report describes a case of severe acute hypocalcaemia in a 64-year-old male with advanced MTC treated with selpercatinib. The patient, who had stable hypoparathyroidism, experienced acute hypocalcaemia (corrected calcium 1.4 mmol/L) 2 weeks after initiating selpercatinib, requiring hospitalisation for calcium supplementation and monitoring. Selpercatinib was temporarily withheld and later reintroduced at a lower dose, successfully preventing recurrence of hypocalcaemia. Investigations excluded other common or important causes of hypocalcaemia, which led us to conclude that this could be a drug-related adverse event. This case highlights the need for careful monitoring of electrolyte disturbances in patients on selpercatinib, particularly those with pre-existing hypoparathyroidism. Although rare, the development of hypocalcaemia with RET inhibitors may necessitate dose interruptions and adjustments. Our experience has also illustrated that re-challenge with selpercatinib is feasible with appropriate management strategies.