Carcinoid tumours are the most commonly occurring endocrine tumours. Although generally found in the gut wall, they also occur in the pancreas, rectum, ovary and lung. Carcinoid tumours typically contain numerous membrane-bound neurosecretory granules, which are composed of a variety of hormones and biogenic amines. One of the best characterized of these substances is serotonin. Serotonin is synthesized from its precursor, 5-hydroxytrytophan, by the enzyme aromatic acid decarboxylase. Serotonin is subsequently metabolised by monoamine oxidase to 5-hydroxyindoleacetic acid, which is excreted in the urine. In addition to serotonin, carcinoid tumours
have been found to secrete corticotropin, histamine, dopamine, substance P, neurotensin, prostaglandins, and kallikrein. Among endocrine tumours of the gastrointestinal tract, carcinoid tumours arising from the midgut secrete serotonin, while carcinoid tumours arising from the hindgut rarely secrete serotonin