A relatively rare condition quite different from DIABETES MELLITUS with which it must not be confused.

It is characterised by excessive thirst and the passing of large volumes of dilute urine. It is due either to a lack of the antidiuretic hormone (VASOPRESSIN) normally produced by the HYPOTHALAMUS and stored in the posterior PITUITARY GLAND, or to a defect in the renal tubules which prevents them from responding to the hormone. When the disorder is due to vasopressin insufficiency, a primary or secondary tumour in the area of the pituitary stalk is responsible for one-third of cases. In another one-third of cases there is no apparent cause, and such IDIOPATHIC cases sometimes run in families. A further one-third of cases result from a variety of lesions including trauma, basal MENINGITIS or a GRANULOMA in the pituitary-stalk area. Treatment is with the analogue of vasopressin called desmopressin which is more potent than the natural hormone and has less effect on blood pressure. It is prescribed as a nasal spray.

When the renal tubules fail to respond to vasopressin this is usually because of a genetic defect transmitted as a sex-linked recessive characteristic, and the disease is called nephrogenic diabetes insipidus. Treatment is with a thiazide diuretic (see THIAZIDES). Metabolic abnormalities such as HYPERCALCAEMIA and POTASSIUM depletion render the renal tubule less sensitive to vasopressin and certain drugs such as lithium and tetracycline may have a similar effect.