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).ntidiuretic 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).