Carcinoid syndrome: A syndrome due to carcinoid tumor which secretes large amounts of the hormone serotonin. The syndrome is directly due to the serotonin. Features include flushing and blushing, swelling of the face (especially around the eyes), flat angiomas (little collections of dilated blood vessels) on the skin, diarrhea, bronchial spasm, rapid pulse, low blood pressure and tricuspid and pulmonary stenosis (narrowing of the tricuspid and pulmonic valves of the heart), often with regurgitation.
Carcinoid tumor usually arises in the gastrointestinal tract, anywhere between the stomach and the rectum (the favorite spot is in the appendix) and from there may metastasize (spread) to the liver. In the liver the tumor produces and releases large quantities of serotonin into the systemic bloodstream.