A 42-year-old man on long-term lithium for bipolar disorder reports polyuria of 7 litres/day and polydipsia. Serum sodium is 146 mEq/L, urine osmolality 95 mOsm/kg. After desmopressin administration, urine osmolality rises to only 105 mOsm/kg. What is the diagnosis?

• A Nephrogenic diabetes insipidus ✓
• B Central diabetes insipidus
• C Primary polydipsia
• D Osmotic diuresis from glycosuria

Explanation

Lithium enters collecting duct principal cells and impairs vasopressin signalling, reducing aquaporin-2 expression, which produces nephrogenic diabetes insipidus. The defining test result is a urine osmolality that fails to rise substantially after desmopressin. In central diabetes insipidus the same stimulus would raise osmolality by more than 50 percent, and in primary polydipsia baseline urine osmolality is usually higher with a brisk desmopressin response.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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