Carcinoid syndrome is caused by a carcinoid tumor that secretes serotonin or other chemicals into your bloodstream. Carcinoid tumors occur most often in the gastrointestinal tract, including your stomach, small intestine, appendix, colon and rectum. Only a small percentage of carcinoid tumors secrete the chemicals that cause carcinoid syndrome.
https://www.mayoclinic.org/diseases-conditions/carcinoid-syndrome/symptoms-causes/syc-20370666

