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Lithium Renal Handling — MRCPsych Paper B MCQ

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HardMood DisordersLithium Renal HandlingMRCPsych Paper B

A 48-year-old woman with bipolar I disorder has maintained a stable 12-hour serum lithium concentration of 0.7 mmol/L on an unchanged dose. Bendroflumethiazide is started for hypertension. Two weeks later, her 12-hour lithium concentration is 1.2 mmol/L despite unchanged serum creatinine and no change in her lithium dose. Which pharmacokinetic mechanism principally explains this rise?

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

Reveal the answer and explanation

Correct answer: A — Reduced renal clearance through enhanced proximal reabsorption after sodium depletion

Explanation lettering: C = shown as A · D = shown as C · A = shown as D

The correct answer is C. Thiazides inhibit distal sodium reabsorption and produce sodium loss. The kidney compensates by increasing proximal tubular sodium reabsorption; lithium is handled similarly to sodium in the proximal tubule, so its reabsorption increases and renal clearance falls. Serum lithium therefore rises even if creatinine remains unchanged. Lithium undergoes no clinically important CYP-mediated metabolism, excluding A, and the interaction is not caused by increased intestinal absorption, excluding B. Lithium is not appreciably protein bound, so displacement cannot explain the increase (D). Extracellular volume contraction may accompany thiazide use, but a change in distribution volume is not the principal mechanism; reduced renal elimination is decisive, excluding E.

Reference: Lithium Carbonate Essential Pharma 250 mg film-coated tablets, Summary of Product Characteristics, sections 4.4, 4.5 and 5.2, revised 2024. https://www.medicines.org.uk/emc/product/10828/smpc