5-hydroxyindoleacetic acid (5-hy-DROK-see-IN-dole-uh-SEE-tik A-sid)
5HIAA. A breakdown product of serotonin that is excreted in the urine. Serotonin is a hormone found at high levels in many body tissues. Serotonin and 5HIAA are produced in excess amounts by carcinoid tumors, and levels of these substances may be measured in the urine to test for carcinoid tumors.