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SIADH — RACP Adult Medicine MCQ

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ModerateMedical OncologySIADHRACP Adult Medicine

A 58-year-old woman undergoing chemotherapy for colon cancer develops a serum sodium of 118 mmol/L. She has been taking cyclophosphamide and vincristine. She is euvolaemic with concentrated urine (osmolality 580 mOsm/kg) and urine sodium of 55 mmol/L. What is the most likely cause of her hyponatraemia?

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Correct answer: C — SIADH

Euvolaemic hyponatraemia with concentrated urine and high urine sodium is consistent with SIADH. Both cyclophosphamide and vincristine are well-recognised causes of SIADH. After excluding hypothyroidism and adrenal insufficiency (which can mimic SIADH), management involves fluid restriction (1–1.5 L/day). Severe symptomatic hyponatraemia may require 3% hypertonic saline.

Reference: eTG – 2025 – Endocrinology; AMH – 2025 – Adverse Drug Reactions