Thiazide-Lithium — MRCPsych Paper B MCQ

Instant feedback + full explanation. One question, done properly.

HardMood DisordersThiazide-LithiumMRCPsych Paper B

A 46-year-old woman maintained on lithium for bipolar affective disorder has a serum lithium concentration of 0.7 mmol/L. Two weeks after a new medicine is started for hypertension, her level rises to 1.4 mmol/L with coarse tremor and nausea. Which medicine most characteristically reduces renal lithium clearance by causing renal sodium loss and extracellular volume contraction, with compensatory increases in proximal tubular sodium and lithium reabsorption?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: E — Bendroflumethiazide

Bendroflumethiazide is a thiazide diuretic. Thiazide-induced natriuresis and mild volume contraction stimulate compensatory proximal tubular sodium reabsorption; because lithium is handled like sodium at that site, renal lithium clearance falls and toxicity can develop within days to weeks, as reflected in UK lithium prescribing information warning of thiazide-precipitated lithium intoxication. Ibuprofen also raises lithium levels, but via inhibition of renal prostaglandin synthesis and reduced renal perfusion, not sodium depletion. Ramipril acts through renin–angiotensin blockade and haemodynamic change in glomerular filtration. Furosemide acts at the loop and is less consistently associated with lithium retention. Acetazolamide, a carbonic anhydrase inhibitor, increases proximal sodium and lithium delivery and therefore tends to enhance lithium excretion.

Reference: Electronic Medicines Compendium (eMC). Lithium Carbonate Essential Pharma 250 mg film-coated tablets — Summary of Product Characteristics, sections 4.4 and 4.5, 2025. https://www.medicines.org.uk/emc/product/10828/smpc