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5-Hydroxy indoleacetic acid (5-HIAA) is the primary metabolite of serotonin. Serotonin is broken down into 5-hydroxy indoleacetic acid within the liver. 5-hydroxy indoleacetic acid is used as a proxy for serotonin measurement through a 24-hour urine test.[1][2] 5-hydroxy indoleacetic acid's primary use is in diagnosing and monitoring carcinoid tumors, a subset of serotonin secreting neuroendocrine tumors.
5-hydroxy indoleacetic acid is essential in diagnosing and monitoring carcinoid tumors. Clinicians need to distinguish carcinoid tumors from carcinoid syndrome. Carcinoid tumors are a form of neuroendocrine tumor within the gastrointestinal or pulmonary system. Midgut carcinoid tumors secrete serotonin, whereas foregut and midgut carcinoid tumors do not. Carcinoid syndrome is a paraneoplastic syndrome secondary to carcinoid tumors with symptoms including but not limited to cutaneous flushing, diarrhea, nausea, vomiting, hepatomegaly, cardiac murmurs, coughing, shortness of breath, and wheezing. Furthermore, 5-hydroxy indoleacetic acid may be an essential biomarker for inborn errors of metabolism, diseases of malabsorption, as well as psychiatric conditions.