Diabetes insipidus is a relatively rare condition and must be diﬀerentiated from DIABETES MELLITUS which is an entirely diﬀerent disease.
It is characterised by excessive thirst and the passing of large volumes of urine which have a low speciﬁc gravity and contain no abnormal constituents. It is either due to a lack of the antidiuretic hormone 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 antidiuretic hormone VASOPRESSIN. 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 are sometimes familial. A further one-third of cases result from a variety of lesions including trauma, basal MENINGITIS and granulomatous lesions in the pituitary-stalk area. 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. 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.
If the disease is due to a deﬁciency of vasopressin, treatment should be with the analogue of vasopressin called desmopressin which is more potent than the natural hormone and has less pressor activity. It also has the advantage in that it is absorbed from the nasal mucosa and so does not need to be injected.
Nephrogenic diabetes insipidus cannot be treated with desmopressin. The urine volume can, however, usually be reduced by half by a thiazide diuretic (see THIAZIDES).