Vasopressin (Anti Diuretic Hormone) acts on the distal tubule and collecting ducts to increase permeability to free water. Deficiency of this hormone in diabetes insipidus results in the excretion of dilute urine (increased water clearance) and hypernatraemia.

In a normal response to the water deprivation test the maximum urine osmolality exceeds plasma osmolality and the urine osmolality does not increase > 5 % after administration of vasopressin. The plasma osmolality normal range is 278-300 mosmol/kg and urine osmolality normal range is 350-1000 mosmol/kg.