Diabetes insipidus is a disorder characterized by the excretion of very dilute urine. As in diabetes mellitus, there is an excessive output of urine accompanied by extreme thirst. Unlike diabetes mellitus, however, there is no sugar in the urine and the blood sugar level is normal.
The substance that normally influences the kidney's capacity to conserve water and form concentrated urine is the antidiuretic hormone called vasopressin. This hormone is secreted by the hypothalamus, a small area of the brain located just above and in front of the pituitary gland. When the hypothalamus or the pituitary gland is diseased or injured, the secretion of vasopressin is decreased or stopped. As a result, large amounts of water are lost from the body, usually 9 liters or more a day. If the patient does not counteract this loss by drinking much water, his blood volume falls, and shock, dehydration, and death may follow.
The known causes of diabetes insipidus include tumors in or near the base of the brain, head injury, and certain brain diseases, such as meningitis or encephalitis. In many cases, however, the cause of the disease is unknown.