Serotonin (5-hydroxytryptamine; 5-HT) is one of the most thoroughly characterized biologically active substance released by Carcinoid tumors. Carcinoids are neuroectodermal tumors of the gastrointestinal tract and pancreas.Slight increases may be seen in acute intestinal obstruction, acute MI, cystic fibrosis, dumping syndromes, and nontropical sprue. Metastasizing abdominal carcinoid tumors often show serotonin concentrations greater than 400 ng/mL. Abstain from medications for 72 hours prior to collection