One of the main secretory products of carcinoid tumors involved in the carcinoid syndrome is serotonin , which is synthesized from tryptophan. Serotonin production is characteristic of gut carcinoid tumors. Serotonin is metabolized to 5-HIAA. Therefore, finding high levels of 5-HIAA in a 24-hour urine collection in a patient with flushing or other symptoms is highly suggestive of the diagnosis. Serotonin has numerous biologic effects, including stimulating intestinal secretion with inhibition of absorption, stimulating increases in intestinal motility, and stimulating fibrogenesis. In various studies 56-88% of all carcinoid tumors were associated with serotonin overproduction; however, 12-26% of the patients did not have the carcinoid syndrome. In one study platelet serotonin was elevated in 96% of patients with midgut carcinoids, 43% with foregut tumors, and 0% with hindgut tumors. In 90-100% of patients with the carcinoid syndrome there is evidence of serotonin overproduction. Serotonin is thought to be predominantly responsible for the diarrhea because of its effects on gut motility and intestinal secretion, primarily through 5-HT3 and, to a lesser degree, 5-HT4 receptors. Ref: Papadakis MA, McPhee SJ, "Carcinoid Syndrome, Malignant." Quick Medical Diagnosis & Treatment ; Jensen R.T. (2012). Chapter 350; Endocrine Tumors of the Gastrointestinal Tract and Pancreas. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison's Principles of Internal Medicine, 18e.