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Vitamin D Deficiency — RACP Adult Medicine MCQ

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ModerateMedical OncologyVitamin D DeficiencyRACP Adult Medicine

A 72-year-old man with metastatic prostate cancer on enzalutamide and androgen deprivation therapy develops fatigue and bone pain. His calcium is 1.85 mmol/L, phosphate is 2.2 mmol/L, and ALP is 450 U/L. Vitamin D is 20 nmol/L. What is the most likely cause of his hypocalcaemia?

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Correct answer: A — Vitamin D deficiency

Vitamin D deficiency is extremely common in patients with metastatic prostate cancer on ADT and is a leading cause of hypocalcaemia in this population. ADT accelerates bone loss, and concurrent vitamin D deficiency exacerbates this. The elevated ALP reflects osteoblastic metastatic disease plus secondary hyperparathyroidism. Correction of vitamin D deficiency with cholecalciferol is essential before commencing bone-protective agents (denosumab or zoledronic acid).

Reference: Cancer Council Australia – 2023 – Prostate Cancer Guidelines; eTG – 2025 – Endocrinology