Also known as: 5-HT, Serum, 5-Hydroxytryptamine, Serum.

Serotonin, a chemical, is derived from the amino acid tryptophan. It is produced as necessary by the nervous system, mainly the brain, but also by special cells in the bronchial tubes (lungs) and gastrointestinal tract. In the blood, most serotonin, more than 90%, is found in the platelets. Serotonin helps constrict blood vessels and transmit nerve impulses, aids in the wake-sleep cycle, and affects mood. It is metabolized by the liver and its metabolites, primarily 5-HIAA (5-hydroxyindoleacetic acid, a muscle stimulant), are then excreted in the urine. Serotonin is normally present in small varying quantities in the blood, and large quantities of serotonin and 5-HIAA may be produced continuously or intermittently by some carcinoid tumors.