SSRI-induced hyponatraemia — MRCPsych Paper A MCQ

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ModeratePsychopharmacologySSRI-induced hyponatraemiaMRCPsych Paper A

A 77-year-old woman develops confusion and recurrent falls three weeks after starting citalopram. Her serum sodium is 124 mmol/L and serum osmolality is low. She is clinically euvolaemic, but her urine remains inappropriately concentrated. Renal, thyroid and adrenal function are normal. Which mechanism most likely accounts for this adverse drug reaction?

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Correct answer: B — Syndrome of inappropriate antidiuretic hormone secretion

The correct answer is B: syndrome of inappropriate antidiuretic hormone secretion (SIADH). SSRIs, including citalopram, can cause hyponatraemia through inappropriate antidiuretic hormone activity; older women are at particularly high risk. The low serum osmolality, clinical euvolaemia and inappropriately concentrated urine support SIADH, while normal renal, thyroid and adrenal function exclude important mimics. Primary polydipsia usually produces very dilute urine. Cerebral salt wasting causes hypovolaemia and is generally associated with intracranial disease. Primary hyperaldosteronism typically causes hypertension and hypokalaemia rather than hypotonic hyponatraemia. Nephrogenic diabetes insipidus causes polyuria with dilute urine and tends towards hypernatraemia; lithium is a classic drug cause.

Reference: electronic Medicines Compendium, Citalopram 10 mg film-coated tablets, Summary of Product Characteristics, section 4.4 Hyponatraemia, updated 18 June 2026. https://www.medicines.org.uk/emc/product/523/smpc