A 55-year-old woman on long-term lithium for bipolar disorder presents with polyuria (5 L/day), polydipsia, and a serum sodium of 148 mEq/L. Urine osmolality after overnight water deprivation is 180 mOsm/kg. After exogenous desmopressin (DDAVP), urine osmolality rises to 600 mOsm/kg. The diagnosis is:

• A Nephrogenic diabetes insipidus from lithium
• B Central (neurogenic) diabetes insipidus ✓
• C Primary polydipsia
• D Osmotic diuresis from hyperglycaemia

Explanation

A rise in urine osmolality to >50% above baseline after DDAVP indicates central diabetes insipidus (CDI) — the kidney can concentrate normally once ADH is supplied exogenously. In nephrogenic DI (more typical of long-term lithium), the kidney is unresponsive to DDAVP and urine osmolality increases by <50% or remains low. The hypernatraemia and dilute urine exclude primary polydipsia.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.