Vasopressin, also known as antidiuretic hormone/ADH, is a "pituitary" hormone which acts to retain water and constrict blood vessels. It increases water reabsorption in the kidney's collecting ducts. Decreased release due to alcohol intoxication or tumor leads to increased blood sodium/excessive urination/thirst. Excess vasopressin leads to low sodium. A number of problems cause inappropriate ADH-tumors, drugs.