Thyroid hormone (triiodothyronine, T3) regulates growth, development and differentiation. To examine the influence of T3 on hepatoma cell growth, thyroid receptor (TR)alpha1 or TRbeta1 over-expressing HepG2 cell lines were used. Growth of the HepG2-TR stable cell line was inhibited by over 50% following treatment with T3. However, transforming growth factor (TGF)-beta neutralizing antibody, but not the control antibody can reverse the cell growth inhibition effect of T3. Flow cytometric analysis indicated that the growth inhibition was apparent at the transition point between the G1 and S phases of the cell cycle. The expression of major cell cycle regulators was used to provide further evidence for the growth inhibition. Cyclin-dependent kinase 2 (cdk2) and cyclin E were down-regulated in HepG2-TR cells. Moreover, p21 protein or mRNA levels were up-regulated by around 5-fold or 7.3-fold respectively following T3 treatment. Furthermore, phospho-retinoblastoma (ppRb) protein was down-regulated by T3. The expression of TGF-beta was studied to delineate the repression mechanism. TGF-beta was stimulated by T3 and its promoter activity was enhanced six- to eight-fold by T3. Furthermore, both T3 and TGF-beta repressed the expression of cdk2, cyclin E and ppRb. On the other hand, TGF-beta neutralizing but not control antibody blocked the repression of cdk2, cyclin E and ppRb by T3. These results demonstrated that T3 might play a key role in liver tumor cell proliferation.