A 22-year-old woman drinks more than 6 litres of water daily. During a supervised water deprivation test, plasma osmolality rises to 298 mOsm/kg while urine osmolality plateaus at 190 mOsm/kg. After administration of desmopressin, her urine osmolality rises to 620 mOsm/kg. What is the most likely diagnosis?
- A Central (cranial) diabetes insipidus ✓
- B Nephrogenic diabetes insipidus
- C Primary (psychogenic) polydipsia
- D Syndrome of inappropriate antidiuretic hormone secretion
Correct answer: A. Central (cranial) diabetes insipidus
Explanation
Inability to concentrate urine during deprivation despite a rising plasma osmolality indicates deficient vasopressin action. C rise in urine osmolality by more than 50 percent after desmopressin confirms that the renal concentrating mechanism is intact, localising the defect to hypothalamic-pituitary ADH secretion, i.e. central diabetes insipidus. In nephrogenic diabetes insipidus there is no significant response to desmopressin, and in primary polydipsia urine concentrates normally during deprivation.
Reference: Williams Textbook of Endocrinology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.