Fig. 5.14 
The causes of polyuria and polydipsia. 
(a) Primary polydipsia is usually related to psychiatric illness. Vasopressin secretion is suppressed by low plasma osmolality induced by the excessive water intake which, if prolonged, may also affect the kidney. (b) Cranial diabetes insipidus is due to a shortage or absence of vasopressin. (c) In nephrogenic diabetes insipidus, there is resistance to the renal action of vasopressin, with a consequent rise in the plasma vasopressin concentration to a level that is inappropriately high relative to urine osmolality.