SIADH (Syndrome of Inappropriate Anti-Diuretic Hormone) occurs when excessive anti-diuretic hormone (ADH; also called vasopressin) is secreted by the posterior pituitary gland. Vasopressin acts on the kidneys to prevent fluid excretion; an excess of the hormone results in water retention and hyponatremia (a low concentration of sodium in the blood).

SIADH may be caused by brain injuries and disorders, hormonal disorders, lung diseases, cancer, infection, and certain medications. Typically, the hyponatremia is treated as well as, when possible, the underlying cause of SIADH.