Carcinoid tumors are tumors that secrete excessive amounts of the hormone serotonin .
Serotonin is converted by the body to 5-HIAA . The precursor to serotonin is tryptophan , an amino acid that is also used by the body to manufacture niacin and certain proteins . In children, carcinoid usually occurs in the appendix and removal of the appendix results in a complete cure. Treatment of the associated tumor reduces the risk of carcinoid syndrome. Signs of pellagra develop when most of the available tryptophan is used by the tumor to produce serotonin , which then prevents the body from manufacturing niacin .