ADH (Anti-Diurtetic Hormone) is a hormone used by your kidneys and determines how much water your body retains or excretes. It is made in the hypothalamus (homeostasis control center of sorts) and this hormone directly effects blood pressure and body water composition.
Think of ADH as a "save water" signal to your kidneys. It is therefore produced in times of water shortage, but not when there is plenty of water in the blood.
So, when there is a shortage of water the pituitary gland increases the production of ADH, and when there is a large concentration of water, ADH production is lowered.
