There are two tests that measure your serotonin levels. The Serotonin Test (also called the 5-HT test) measures the amount of serotonin in your blood, and the 5-HIAA test measures the amount of 5-hydroxyindoleacetic acid (5-HIAA) in the urine. 5-HIAA is a breakdown product of serotonin and indicates raised levels of serotonin. While there is a test that measures serotonin in your blood, it’s generally only used to check for serotonin-producing tumors outside of the brain. Also, blood serotonin levels don’t necessarily reflect the levels in your brain.