because increased thirst stimulates oral consumption of fluids, which maintains the serum sodium near the upper normal range as long as access to fluids is not impaired. In a water restriction (or deprivation) test, urine volume, urine osmolality, and plasma sodium concentration are measured hourly after complete water restriction. A normal urine osmolality response (usually defined as an increase in urine osmolality above 600 mOsm/kg H 2 O) indicates that ADH release and corresponding renal response to ADH are intact. A failure of the urine osmolality to rise despite rising plasma osmolality suggests either central or nephrogenic DI. Desmopressin is then administered. Patients with central DI will respond with increased urine osmolality, whereas in patients with nephrogenic DI (as is likely in this patient), desmopressin will not result in increased urine osmolality after water restriction, confirming the diagnosis.