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Lithium NDI — MRCPsych Paper B MCQ

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HardMood DisordersLithium NDIMRCPsych Paper B

A 50-year-old man with bipolar I disorder has taken lithium 1200 mg daily for 8 years. His current 12-hour serum lithium concentration is 0.7 mmol/L. He reports progressive thirst and passes approximately 6 litres of urine daily. Serum sodium is 148 mmol/L, plasma osmolality is 306 mOsm/kg and urine osmolality is 160 mOsm/kg. Following supervised administration of desmopressin, his urine osmolality remains essentially unchanged. Which diagnosis best accounts for these findings?

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Correct answer: B — Lithium-induced nephrogenic diabetes insipidus

The correct answer is **lithium-induced nephrogenic diabetes insipidus**. Marked polyuria, hypernatraemia and raised plasma osmolality accompanied by inappropriately dilute urine indicate failure of renal water conservation. The negligible response to desmopressin shows renal resistance to vasopressin, distinguishing nephrogenic from central diabetes insipidus, in which urine osmolality should rise. Lithium is a well-established cause and the complication may occur despite a therapeutic serum lithium concentration. Primary polydipsia generally lowers plasma osmolality and may produce hyponatraemia rather than hypernatraemia. Primary adrenal insufficiency and SIADH also characteristically cause hyponatraemia with urine that is not maximally dilute, so neither explains this hyperosmolar water diuresis.

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