The glucuronidation of thyroid hormone by UDP-glucuronyltransferases (UGTs) stably transfected in Chinese hamster V79 lung fibroblasts was investigated. Human bilirubin UGT (HP3) and phenol UGT (HP4) both catalysed the glucuronidation of T4 and rT3, whereas glucuronidation of T3 was not significant, rT3 was the preferred substrate for both isoenzymes, glucuronidation rates being 1.6- and 6.4-times higher than conjugation of T4 by HP3 and HP4 clones, respectively. This is the first identification of thyroid hormone as potential alternative endogenous substrate for bilirubin UGT.