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SSRI SIADH Elderly — MRCPsych Paper B MCQ

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ModerateLiaison PsychiatrySSRI SIADH ElderlyMRCPsych Paper B

A 74-year-old woman develops confusion 3 weeks after starting sertraline for a depressive episode. Her serum sodium is 119 mmol/L and serum osmolality is 260 mOsm/kg. She is clinically euvolaemic. Urine osmolality is 420 mOsm/kg and urine sodium is 48 mmol/L. Blood glucose, renal, hepatic and thyroid function are normal, and a short Synacthen test shows an adequate cortisol response. She is not taking a diuretic. What is the most likely diagnosis?

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Correct answer: B — Sertraline-induced syndrome of inappropriate antidiuretic hormone secretion

The answer is B: sertraline-induced SIADH. The patient has symptomatic hypotonic hyponatraemia, clinical euvolaemia and urine that remains inappropriately concentrated, with a urine sodium above 30 mmol/L. Recent sertraline exposure and older age strengthen the causal attribution; UK product information confirms that SSRI-associated hyponatraemia commonly reflects SIADH and that older people are at greater risk. Primary adrenal insufficiency can mimic SIADH but is excluded by the adequate Synacthen response. Diabetes insipidus causes polyuria with dilute urine, usually with normal or raised sodium. Primary polydipsia produces appropriately dilute urine. Diabetic ketoacidosis requires hyperglycaemia and typically causes hypertonic or translocational rather than true hypotonic hyponatraemia.

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