Disorder in the regulation of the body’s salt and water content, caused by antidiuretic hormone deficiency as a result of a disturbance in the pituitary gland. In order to keep a constant balance of salt and water in the body the kidneys produce large or small quantities of urine, containing salts and waste products in solution. The quantity of water which leaves the body in this way is regulated by a hormone produced by the pituitary gland, which inhibits the secretion of water – hence its name antidiuretic hormone (ADH). In diabetes insipidus the production of this hormone is disturbed. The most important cause of the deficiency is an injury to the skull. A tumour of the pancreas can also result in lowered hormone production. Sometimes no cause at all can be found. Up to 20 litres of water can be lost in the urine within 24 hours, causing unquenchable thirst. Other symptoms are associated with constant loss of fluid, such as a dry throat and constipation. There is a danger of dehydration, particularly if a patient is bedridden and not enough fluid can be provided. A thirst test is sometimes used to confirm diagnosis; the patient is not allowed to drink, and urine production is measured In diabetes insipidus the patient discharges many litres of urine per day because the pituitary gland is not producing enough antidiuretic hormone to allow the kidneys to balance the body’s fluids. Over a period of hours. Under normal circumstances ADH ensures that no more urine is produced, but in patients with diabetes insipidus too much urine continues to be produced; if after an ADH injection urine production is lowered, the diagnosis is certain. Treatment is by removal of any tumour and administration of an artificial ADH preparation. This can be by injection, but there are also preparations that can be placed on the nasal mucous membrane and inhaled. This treatment usually has to be maintained throughout the patient’s lifetime. If brain damage is the cause the condition may be temporary, and the pancreas will later produce sufficient ADH, when treatment can stop.