A 53 year old woman presents with excessive thirst and increased frequency of urination for 3 months. She sleeps poorly as she has to urinate at least twice each night. Her medical, surgical and family histories are unremarkable. A fasting plasma glucose is found to be 80 mg/dL. Her full blood count, renal functions, serum sodium, potassium, chloride and calcium levels, and serum uric acid levels are within normal parameters.
Her urine output is found to be 4L/24h, with a specific gravity of 1.000 (normal: 1.002-1.030).
The random urine osmolality is 106 mosmol/kg (normal: 300-900)
After 12 hours of fluid deprivation the urine osmolality is 152 mosmol/kg (normal: >600) A desmopressin injection is administered and the test repeated; following this, the urine osmolality is 846 mosmol/kg
Absence of the normal posterior pituitary hyperintensity is noted in the T1 weighted images. The remainder of the pituitary gland and the pituitary stalk appear normal. No other abnormalities are noted.