A 22-year-old man presents with polyuria and polydipsia. During a supervised water deprivation test, urine osmolality plateaus at 180 mOsm/kg despite a plasma osmolality of 302 mOsm/kg. Administration of desmopressin produces no rise in urine osmolality. What is the most likely diagnosis?

• A Central diabetes insipidus
• B Syndrome of inappropriate ADH secretion
• C Primary polydipsia
• D Nephrogenic diabetes insipidus ✓

Explanation

In nephrogenic diabetes insipidus the renal tubules cannot respond to ADH, so urine osmolality fails to concentrate during dehydration and remains flat after desmopressin. In central diabetes insipidus, desmopressin raises urine osmolality by more than 50 percent, and in primary polydipsia the kidney concentrates normally after deprivation. A rising plasma osmolality with persistently dilute urine excludes SIADH, where urine is inappropriately concentrated.

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

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Written and medically reviewed by the StethoPrep medical team.