Lithium Side Effects — MRCPsych Paper B MCQ

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ModerateMood DisordersLithium Side EffectsMRCPsych Paper B

A 50-year-old man with bipolar I disorder has taken lithium for 8 years. He develops persistent polydipsia and passes 5 litres of urine daily. His 12-hour serum lithium concentration is 0.7 mmol/L, plasma glucose is normal and eGFR is 90 mL/min/1.73 m². Plasma osmolality is 301 mOsm/kg and urine osmolality is 120 mOsm/kg. What is the most likely cause?

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Correct answer: C — Lithium-induced nephrogenic diabetes insipidus

The correct answer is C, lithium-induced nephrogenic diabetes insipidus, now also termed acquired arginine vasopressin resistance. Long-term lithium can impair the renal collecting ducts' response to arginine vasopressin, causing high-volume dilute urine despite a high-normal plasma osmolality. This adverse effect can occur while the serum lithium concentration is within the therapeutic range. Diabetes mellitus is unlikely because plasma glucose is normal. Lithium-induced chronic kidney disease may impair urinary concentration, but the normal eGFR argues against clinically significant CKD as the principal cause. Primary polydipsia usually lowers plasma osmolality rather than producing a high-normal value with markedly dilute urine. Hyperthyroidism can cause thirst but does not characteristically cause this paired osmolality pattern.

Reference: Electronic Medicines Compendium. Lithium Carbonate Essential Pharma 250 mg film-coated tablets, Summary of Product Characteristics, sections 4.4 and 4.8. Updated 21 January 2025. https://www.medicines.org.uk/emc/product/10828/smpc