Uridine 5'-diphospho-glucuronosyltransferases (UGTs) are phase II enzymes foundin the liver and are responsible for glucuronidation of metabolites to facilitatesystemic elimination.• Circulating concentrations of thyroid hormone (TH) are decreased throughinduction of thyroxine (T4) glucuronidation by many xenobiotics which furtherleads to increased secretion of thyroid-stimulating hormone, causing increasedthyroid follicular cell proliferation, hypertrophy and ultimately follicular celladenomas/carcinomas (Figure-1).• This disruption is due to an increased expression and activity of UGTs particularlyinvolved in the glucuronidation of T4.• An in vitro comparative UGT assay showing that UGT is induced in rodents but notin humans could establish that any subsequent key events and adverse outcomesassociated with this mechanism of action will not occur in humans and thus arenot considered relevant to humans due to differences in TH physiology.• An in vitro method for primary rat and human hepatocytes was developed forassessment of induction of UGT gene expression and enzyme activity. LC-MS/MSassays were developed and validated for the simultaneous determination ofThyroxine (T4) and Thyroxine-o-β-D-Glucuronide (T4-Gluc) concentrations in bothrat and human hepatocyte media supernatant samples, to assess UGT enzymeactivity.