Serotonin-d4 (5-Hydroxytryptamine-d4) hydrochloride is the deuterium labeled Serotonin (hydrochloride) (HY-B1473)[1]. Serotonin (5-Hydroxytryptamine) hydrochloride is a monoamine neurotransmitter in the CNS and an endogenous 5-HT receptor agonist. Serotonin hydrochloride is also a catechol O-methyltransferase (COMT) inhibitor with a Ki of 44 µM[2].