what causes carcinoid tumors
Causes. Carcinoid syndrome is caused by a carcinoid tumor that secretes serotonin or other chemicals into your bloodstream. Carcinoid tumors occur most commonly in your gastrointestinal tract, including your stomach, small intestine, appendix, colon and rectum, or in your lungs. Only a small percentage of carcinoid tumors secrete the chemicals that cause carcinoid syndrome.