A 19-year-old man with polyuria (6 L/day) and polydipsia undergoes a water deprivation test. After seven hours his urine osmolality plateaus at 130 mOsm/kg. He receives 5 micrograms of desmopressin subcutaneously, and one hour later urine osmolality rises to 480 mOsm/kg. Serum sodium at the end is 148 mEq/L. What is the diagnosis?

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

Explanation

Failure to concentrate urine during dehydration with a rise of more than 50 percent in urine osmolality after desmopressin establishes central diabetes insipidus. In nephrogenic diabetes insipidus the kidney cannot respond, so desmopressin produces less than a 50 percent increase. In primary polydipsia the patient concentrates urine normally during deprivation, and SIADH is incompatible with hypernatraemia and dilute urine. The discriminating value is the desmopressin response, not the baseline osmolality.

Reference: Williams Textbook of Endocrinology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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