Antidiuretic hormone (ADH): A relatively small (peptide) molecule that is released by the pituitary gland at the base of the brain after being made nearby (in the hypothalamus). ADH has an antidiuretic action that prevents the production of dilute urine (and so is antidiuretic).

A syndrome of inappropriate secretion of ADH results in the inability to put out dilute urine, perturbs fluid (and electrolyte) balance, and causes nausea, vomiting, muscle cramps, confusion and convulsions. This syndrome may occur in association with oat-cell lung cancer, pancreatic cancer, prostate cancer, and Hodgkin's disease as well as a number of other disorders.

ADH also can stimulate contraction of arteries and capillaries. ADH is also known as vasopressin.