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Carbamazepine-Induced SIADH — RACP Adult Medicine MCQ

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ModerateNeurologyCarbamazepine-Induced SIADHRACP Adult Medicine

A 45-year-old man on carbamazepine for epilepsy presents with Na⁺ 125 mmol/L. He is euvolaemic. Serum osmolality 262 mOsm/kg. Urine osmolality 380 mOsm/kg. Urine Na⁺ 45 mmol/L. The hyponatraemia developed gradually over 3 months. What is the most likely cause?

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Correct answer: A — Carbamazepine-induced SIADH

Euvolaemic hyponatraemia with SIADH pattern (low serum osm, inappropriately concentrated urine, high urine Na⁺) on carbamazepine is drug-induced SIADH. Carbamazepine (and its derivative oxcarbazepine) stimulates ADH release and potentiates its action. Incidence is ~5-40% (more common with oxcarbazepine). Management: fluid restriction, reduce/switch antiepileptic if possible (levetiracetam, lamotrigine have lower risk).

Reference: AMH – 2025 – Carbamazepine; European Hyponatraemia Guidelines 2023