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Paraneoplastic SIADH — RACP Adult Medicine MCQ

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EasyGeneral Internal MedicineParaneoplastic SIADHRACP Adult Medicine

A 65-year-old man with a 40-pack-year smoking history presents with weight loss and a cough. CXR shows a central mass. Serum sodium is 120 mmol/L. Urine osmolality is 500 mOsm/kg and urine sodium is 60 mmol/L. What is the most likely cause of his hyponatraemia?

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Correct answer: C — Thiazide diuretic use

Central lung mass with hyponatraemia and concentrated urine in a heavy smoker is classic for SIADH due to ectopic ADH secretion from small cell lung cancer (SCLC). SCLC is the most common malignancy causing SIADH. Other paraneoplastic syndromes associated with SCLC include Cushing syndrome (ectopic ACTH), Lambert-Eaton myasthenic syndrome, and cerebellar degeneration. Treatment of hyponatraemia includes fluid restriction and definitive cancer treatment.

Reference: eTG – 2025 – Endocrinology; Cancer Council Australia – 2021 – Lung Cancer OCP