SULT6B1 is a cytosolic sulfotransferase that catalyzes phase II conjugation reactions using 3'-phosphoadenosine 5'-phosphosulfate (PAPS) as a sulfate donor 1. The enzyme exhibits sulfation activity toward thyroxine and bithionol 2, with an optimal pH of 8.0 2. SULT6B1 belongs to the SULT6 gene family and represents a structurally distinct group within the sulfotransferase superfamily 3. The enzyme was first functionally characterized through proluciferin sulfation assays and molecular modeling studies demonstrating catalytic competence with PAPS binding 1. Tissue-specific expression patterns differ between species; human SULT6B1 is expressed in kidney and testis 2, while expression appears regulated across multiple tissues in mice 2. SULT6B1 does not catalyze sulfation of certain xenobiotics including 12-hydroxynevirapine and tolvaptan 45, indicating substrate selectivity. A genome-wide association study identified SULT6B1 as a locus showing association with schizophrenia risk, though this finding requires further validation 6. The precise physiological role of SULT6B1 in thyroxine metabolism and additional substrate specificity remain incompletely characterized.