Thiazide-induced Hyponatraemia — EECC MCQ

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ModerateHypertension & Preventive CardiologyThiazide-induced HyponatraemiaEECC

A 48-year-old man with hypertension is started on chlorthalidone 12.5 mg. After 4 weeks, his sodium drops to 128 mmol/L and he develops confusion. What is the most likely diagnosis?

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Correct answer: A — Thiazide-induced hyponatraemia — a well-recognised adverse effect, particularly in elderly women, caused by impaired free water excretion and SIADH-like effect

Thiazide-induced hyponatraemia is a common and potentially serious adverse effect, occurring in approximately 5-10% of patients on thiazide/thiazide-like diuretics. Risk factors include: older age, female sex, low body weight, low dietary sodium intake, and concomitant medications (SSRIs, carbamazepine). The mechanism involves impaired diluting capacity of the distal nephron (thiazides block sodium reabsorption in the DCT, reducing the ability to excrete free water) combined with continued ADH-mediated water reabsorption. Management: stop the thiazide, fluid restrict if symptomatic, correct sodium slowly (max 8-10 mmol/L per 24h to avoid osmotic demyelination). Severe hyponatraemia (<120 mmol/L) or neurological symptoms may require hypertonic saline. Thiazides should generally not be re-prescribed after an episode.

Reference: ESC (2024): Hypertension Guidelines; BNF