Notes: 5-hydroxy Indole-3-acetic acid (5-HIAA) is a metabolite of the monoamine neurotransmitter serotonin (5-HT; Item No. 14332).{40084} It is formed from serotonin via a 5-hydroxy indole-3-acetaldehyde intermediate by aldehyde dehydrogenase. 5-HIAA has been used as a biomarker for the detection of neuroendocrine tumors. Brain levels of 5-HIAA are decreased in patients with severe Alzheimer’s disease.{69100}