antidiuretic hormone (ADH), is a neurohypophysial hormone found in mostmammals. Its two primary functions are to retain water in the body and to constrict blood vessels. The antidiuretic hormone (ADH) test is ordered by itself or along with or following other tests to help detect, diagnose, and determine the cause of antidiuretic hormone deficiencies and excesses. Symptoms of ADH deficiency are often seen with one of two types of diabetes insipidus. Central diabetes insipidus is a decrease in the production of ADH by the hypothalamus or in the release of ADH from the pituitary; nephrogenic diabetes insipidus is characterized by a decrease in the kidney's response to ADH. A water deprivation ADH stimulation test is sometimes used to distinguish between these types. It involves fluid restriction, an ADH test, and the administration of ADH (vasopressin).