SSRI-Induced SIADH — MRCPsych Paper B MCQ

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ModerateMood DisordersSSRI-Induced SIADHMRCPsych Paper B

A 65-year-old man with depression develops confusion three weeks after starting sertraline. His serum sodium is 125 mmol/L and serum osmolality is 265 mOsm/kg. He is clinically euvolaemic. Urine osmolality is 450 mOsm/kg and urine sodium is 55 mmol/L. Blood glucose, renal function, thyroid function and morning cortisol are normal. Which condition most likely explains these findings?

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Correct answer: E — SIADH

The answer is E, SIADH, most likely induced by sertraline. The patient has hypotonic hyponatraemia despite inappropriately concentrated urine, raised urine sodium and clinical euvolaemia. Normal cortisol, thyroid and renal function exclude important mimics. SSRIs, including sertraline, can cause SIADH-associated hyponatraemia, with older adults at increased risk. Psychogenic polydipsia would usually suppress antidiuretic hormone and produce very dilute urine. Diabetes insipidus causes dilute polyuria and usually normal or raised serum sodium rather than concentrated urine with hyponatraemia. Uncontrolled diabetes mellitus can cause hypertonic hyponatraemia, but the glucose and measured serum osmolality do not support this. Addison's disease may resemble SIADH biochemically but is excluded here by the normal morning cortisol. Symptomatic hyponatraemia requires urgent medical assessment and consideration of stopping sertraline.

Reference: Electronic Medicines Compendium. Sertraline 100 mg Tablets: Summary of Product Characteristics, sections 4.2 and 4.4 (Hyponatraemia), updated 5 June 2026. https://www.medicines.org.uk/emc/product/5759/smpc