Definition

A substance formed by the breakdown of a hormone called serotonin in the liver. Nerves use serotonin to send messages to one another. After 5-HIAA is broken down from serotonin in the liver, the kidneys filter it from the blood and pass it into the urine. Serotonin and 5-HIAA may be found in higher-than-normal amounts in patients with certain tumors, such as carcinoid tumor. Measuring the amount of 5-HIAA in the urine may help diagnose and monitor carcinoid tumors and other conditions. 5-HIAA is a type of tumor marker. Also called 5-hydroxyindoleacetic acid.