5-Hydroxyindoleacetic acid (5 HIAA) is the main metabolite of serotonin in the human body. In chemical analysis of urine samples, 5 HIAA is used to determine the body's levels of serotonin. A serotonin analysis is most frequently performed for the diagnosis of carcinoid tumors of the enterochromaffin (Kultschitzsky) cells of the small intestine. These tumor cells release large amounts of serotonin, which can produce the clinical syndrome of flushing, diarrhea, and right-sided heart failure referred to as Serotonin syndrome.