Serotonin or 5-Hydroxytryptamine (5-HT) is a monoamine neurotransmitter derived from the amino acid tryptophan. It is produced as needed by the nervous system, mainly the brain, but also by special cells in the bronchial tubes (lungs) and gastrointestinal tract. Serotonin helps transmit nerve impulses and constrict blood vessels, is a participant in the wake-sleep cycle, and affects mood, appetite, muscle contraction, and some cognitive functions including memory and learning. Serotonin is metabolized by the liver and its metabolites, primarily 5-HIAA (5-hydroxyindoleacetic acid, a muscle stimulant), are excreted in the urine. Large quantities of serotonin and 5-HIAA may be produced continuously or intermittently by some carcinoid tumors. Therefoe, this test is used to help diagnose carcinoid tumors. Any disruption in the synthesis, metabolism or uptake of serotonin has been found to be partly responsible for certain manifestations of schizophrenia, depression, compulsive disorders and learning problems. INCREASED levels are mostly associated with carcinoid tumors. While DECREASED serotonin levels are found in Down’s syndrome, untreated phenylketonuria, Parkinson’s disease, and severe depression. See 5-Hydroxy-Indole Acetic Acid (5-HIAA), 24 Hrs Urine