SSRI-Induced SIADH — MRCPsych Paper B MCQ

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ModerateOld Age PsychiatrySSRI-Induced SIADHMRCPsych Paper B

A 78-year-old man develops new confusion and unsteadiness 3 weeks after starting sertraline 50 mg daily. He is clinically euvolaemic and is not taking a diuretic. Investigations show serum sodium 124 mmol/L, serum osmolality 265 mOsm/kg, urine osmolality 420 mOsm/kg and urine sodium 48 mmol/L. Renal function, thyroid function and a 9 am cortisol are normal. Which diagnosis best explains this presentation?

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Correct answer: E — SSRI-induced syndrome of inappropriate antidiuretic hormone secretion

The correct answer is **B: SSRI-induced SIADH**. The patient has hypotonic hyponatraemia with clinical euvolaemia and urine that remains inappropriately concentrated, including a urine sodium above 30 mmol/L. Normal renal, thyroid and adrenal investigations help exclude important alternative causes. Older people are particularly susceptible to SSRI-associated hyponatraemia, and confusion and unsteadiness are recognised manifestations. Serotonin syndrome would usually include autonomic and neuromuscular abnormalities such as hyperthermia, diaphoresis, clonus or hyperreflexia. Overdose is unsupported by the history and does not specifically explain this biochemical pattern. Discontinuation syndrome follows dose reduction or cessation, not ongoing treatment. A hypersensitivity reaction would be suggested by features such as urticaria, angioedema or anaphylaxis. Symptomatic hyponatraemia requires urgent medical assessment, consideration of stopping sertraline and appropriately controlled correction.

Reference: electronic Medicines Compendium, Sertraline 100 mg film-coated tablets, Summary of Product Characteristics, sections 4.2 and 4.4, updated 5 June 2026. https://www.medicines.org.uk/emc/product/5759/smpc