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SSRI-Induced SIADH — MRCPsych Paper B MCQ

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ModerateMood DisordersSSRI-Induced SIADHMRCPsych Paper B

A 65-year-old man is started on sertraline for a depressive episode. Three weeks later, he becomes mildly confused. He is clinically euvolaemic and has a serum sodium concentration of 126 mmol/L and serum osmolality of 266 mOsm/kg. Urine osmolality is 420 mOsm/kg and urine sodium is 55 mmol/L. Renal, thyroid and adrenal function are normal, and he is not taking a diuretic. Which mechanism best explains these findings?

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Correct answer: D — Inappropriately sustained ADH activity causing renal free-water retention

The answer is D. New hypotonic, euvolaemic hyponatraemia after starting sertraline, with inappropriately concentrated urine and elevated urine sodium despite normal renal, thyroid and adrenal function, is characteristic of SIADH. Continued ADH activity increases collecting-duct water reabsorption, producing dilutional hyponatraemia; the confusion indicates symptomatic hyponatraemia. The sertraline SmPC confirms that SSRI-associated hyponatraemia frequently results from SIADH and that older people are at greater risk. Renal failure is excluded and would not specifically produce this euvolaemic pattern. Primary adrenal insufficiency is excluded by normal adrenal function and may cause hypovolaemia and hyperkalaemia. Diabetes insipidus causes dilute polyuria and tends towards hypernatraemia. Primary polydipsia normally suppresses ADH, resulting in very dilute rather than concentrated urine.

Reference: Sertraline 100 mg Tablets, Summary of Product Characteristics, section 4.4: Hyponatraemia, updated 5 June 2026. https://www.medicines.org.uk/emc/product/5759/smpc