Serotonin (5-hydroxytryptamine) is a monoamine neurotransmitter synthesized from the amino acid tryptophan. Most of total body serotonin is found in the gastrointestinal (GI) tract, where it modulates the functions of the bowel by controlling smooth muscle contraction. It also acts as a cardiovascular vasoconstriction mediator and in the brain as a neurotransmitter. Neuroendocrine neoplasms (NENs), which originate from GI tract enterochromaffin cells, may produce large amounts of serotonin.