5-HIAA (5-hydroxyindoleacetic acid) is a metabolite of the neurotransmitter serotonin. It is produced when serotonin is broken down in the body, and it is excreted in the urine. High levels of 5-HIAA in the urine can indicate an overproduction of serotonin, which is associated with a number of mental health disorders, such as depression and anxiety. Low levels, on the other hand, can indicate an underproduction of serotonin, which can lead to poor impulse control and other psychological issues. Measuring the levels of 5-HIAA in urine can help to diagnose and treat certain mental health conditions.