Triiodothyronine (T3), through interaction with its intracellular thyroid hormone receptors (TRs), influences various physiological functions, including metabolism, development, and growth. We investigated the effect of T3 and the selective TR-β agonist GC-1 in two models of hepatocarcinogenesis. Preneoplastic lesions were induced in F-344 rats via a single dose of diethylnitrosamine, followed by a choline-deficient (CD) diet for 10 weeks. Rat subgroups were then fed the CD diet or a CD diet containing either 4 mg/kg T3 or 5 mg/kg GC-1 for another week. Rats fed a CD diet alone showed a large number (65/cm2) of preneoplastic lesions positive for the placental form of glutathione S-transferase (GSTP). Coadministration of T3 for the last week caused an almost complete disappearance of the foci (3/cm2). A reduction of GSTP-positive foci was also observed in rats fed a CD + GC-1 diet (28/cm2 versus 75/cm2 of rats fed a CD diet alone) in the absence of significant differences in labeling or apoptotic index of preneoplastic hepatocytes between the two groups. An antitumoral effect of GC-1 was also observed with the resistant hepatocyte model of hepatocarcinogenesis. Nodule regression was associated with a return to a fully differentiated phenotype, indicated by the loss of the fetal markers GSTP and gamma glutamyl transpeptidase, and reacquisition of the activity of glucose 6-phosphatase and adenosine triphosphatase, two enzymes expressed in normal hepatocytes. Conclusion: Our results indicate that activated TRs negatively influence the carcinogenic process through induction of a differentiation program of preneoplastic hepatocytes. The results also suggest that TRs could be a meaningful target in liver cancer therapy. (HEPATOLOGY 2009.)