Lithium-Induced NDI — RACP Adult Medicine MCQ

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EasyEndocrinology & DiabetesLithium-Induced NDIRACP Adult Medicine

A 65-year-old man on long-term lithium for bipolar disorder presents with polyuria (6 L/day), polydipsia, and a serum sodium of 148 mmol/L. His serum osmolality is 305 mOsm/kg and urine osmolality is 180 mOsm/kg. After DDAVP administration, his urine osmolality does not concentrate (remains <300 mOsm/kg). What is the diagnosis?

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Correct answer: A — Nephrogenic diabetes insipidus

Lithium is the most common cause of acquired nephrogenic diabetes insipidus (NDI). It inhibits vasopressin (ADH) action on renal collecting duct aquaporin-2 channels by inhibiting the cAMP signalling pathway. The key diagnostic finding is failure to concentrate urine after DDAVP administration (excluding central DI). Central DI shows a brisk response to DDAVP. Treatment includes amiloride (which blocks lithium entry into principal cells), thiazide diuretics (paradoxically reduce urine volume), and adequate hydration.

Reference: eTG – 2025 – Endocrinology; AMH – 2025 – Lithium