5-HIAA is a metabolite of:

Serotonin is conveed to 5 HIAA by enzyme monoamine oxidase. In humans consuming a normal diet, the excretion of 5-HIAA is used as a measure of serotonin synthesis. And therefore its excretion in 24 hours is used as a diagnostic test for tumors such as carcinoids which synthesize excessive quantities of serotonin. Serotonin is synthesised from tryptophan by hydroxylation of the indole ring followed by decarboxylation of the amino acid. Serotonin is present in highest concentration in the platelets and in GIT. In GIT its present in the enterochromaffin cells and the myenteric plexus. Ref: Katzung B.G. (2012). Chapter 16. Histamine, Serotonin, & the Ergot Alkaloids. In B.G. Katzung, S.B. Masters, A.J. Trevor (Eds), Basic & Clinical Pharmacology, 12e.