Here we examined the in vivo effects of exposure to a potent in vitro inhibitor of MCT8, the flavonolignan silychristin, on several aspects of the TH system. Adult female rats were daily gavaged with 0, 250, or 500 mg/kg/day (n = 10/group) of silychristin for 7 days and euthanized on day 8. A smaller group (n = 5/group) of rats was administered the related flavonolignan, silybin (900 mg/kg), or the milk-thistlederived flavonolignan mixture, silymarin (1,500 mg/kg). Serum TH concentrations were not changed in any treatment group. Mct8 and Oatp1c1 expression were upregulated in the choroid plexus upon silymarin exposure, without change in response to silychristin or silybin. Deiodinase 1 and dehalogenase activities, unchanged in the liver, were increased in the thyroid by the high dose of silychristin. This dataset is associated with the following publication: Renko, K., R. Thomas, M. Hawks, J. Ford, J. Köhrle, M. Axelstad, and M.E. Gilbert. Examining in vivo effects of silychristin, a potent in vitro inhibitor of thyroid hormone transporter MCT8. Frontiers in Toxicology. Frontiers, Lausanne, SWITZERLAND, 8: 1796387, (2026).