A 52-year-old man on lithium carbonate for bipolar disorder reports passing 7 litres of urine daily with intense thirst. Serum sodium is 149 mEq/L. After controlled water deprivation, urine osmolality reaches only 115 mOsm/kg. One hour after subcutaneous desmopressin, urine osmolality is 128 mOsm/kg. What is the diagnosis?
- A Central diabetes insipidus
- B Primary polydipsia
- C Nephrogenic diabetes insipidus secondary to lithium ✓
- D Osmotic diuresis from undetected diabetes mellitus
Correct answer: C. Nephrogenic diabetes insipidus secondary to lithium
Explanation
Lithium enters collecting duct principal cells and impairs aquaporin-2 expression and vasopressin signalling, producing nephrogenic diabetes insipidus. The defining test result is failure of urine osmolality to rise by more than roughly 50 percent after desmopressin, as seen here. In central diabetes insipidus, desmopressin would raise osmolality substantially, and in primary polydipsia the baseline concentrating ability is preserved once water intake is withheld.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.