SEROTONIN METABOLITE ASSAY - This is a test which measures the amount of 5-HIAA, a metabolite of serotonin, in the urine. A 24 hour urine collection is required to perform this assay. Serotonin is a neurotransmitter that is synthesized from the amino acid tryptophan by enterochromaffin cells in the gut and bronchi. It is metabolized to 5-HIAA in the liver and then excreted in the urine. Elevations in 5-HIAA can indicate carcinoid tumor. The normal range in a 24 hour urine collection is 3 to 15 mg/24 hours.