SULT1A4 is a cytosolic sulfotransferase that catalyzes sulfate conjugation of neurotransmitters and drugs. The enzyme utilizes 3'-phospho-5'-adenylyl sulfate (PAPS) as a sulfonate donor to metabolize phenolic monoamines including dopamine, epinephrine, norepinephrine, and serotonin 1. SULT1A4 also sulfates thyroid hormones with substrate preference of 3,3'-T2 > rT3 > T3 > T4 2. The gene is functionally active and duplicated with SULT1A3 on chromosome 16.2 2. Genetic polymorphisms significantly alter enzyme kinetics; missense variants show differential sulfating activity toward analgesic drugs (acetaminophen, morphine, tapentadol), sympathomimetics (phenylephrine, salbutamol), and neurotransmitters 341. Copy number variation of SULT1A3/4 genes ranges from 1-10 copies and associates with neurodegenerative disease risk: Alzheimer's disease patients show significantly lower copy numbers with correlation to earlier disease onset 5. These pharmacogenetic variations may influence individual drug metabolism, toxicity profiles, and neurotransmitter homeostasis relevant to neurological disease susceptibility.