5-Hydroxyindole-3-acetic acid (5-HIAA) is the catabolic end product of serotonin pathway. 5-HIAA is present in body fluids like cerebrospinal fluid (CSf) blood and urine. The expression levels of 5-HIAA is different among the normal and cancer patients. Sepiapterin reductase deficiency results in low level of 5-HIAA in CSf. 5-HIAA level is low in CSf in patients with bipolar 1 disorder with childhood attention-deficit hyperactivity disorder (ADHD).