5-Hydroxyindole is a hydroxylated indole used as a building block in the preparation of various pharmaceutical compounds such as indole based neurochemicals. 5-Hydroxyindole is a metabolite of Tryptophan . 5-Hydroxyindole displayed weak inhibitory activity on human melanoma tyrosinase. 5-Hydroxyindole also showed inhibition of serotonin transport by blood platelets.