5-HIAA is a serotonin metabolite, which is needed by the nervous system and by special cells in the lung and gastrointestinal tract. After serotonin is used by the body, it is degraded broken down to its metabolites, e.g., 5-HIAA—which is excreted in the urine. In a normal person, only small amounts of 5-HIAA are present in the urine, whereas large amounts are found in cases of carcinoid tumors and neuroendocrine tumors.