Transforming growth factor beta (TGF beta) is a secreted polypeptide factor that is thought to play a major role in the regulation of proliferation of many cell types and various differentiation processes. TGF beta acts on thyroid cells by inhibiting cell proliferation and expression of differentiation markers, such as thyroglobulin production and iodide uptake. Exponentially growing thyroid cells cultures accumulate mostly in G0/G1 after exposure to TGF beta for 48 hours. TGF beta inhibits the TSH induced transition of quiescent thyroid cell from the G0 to the S phase. These effects on the thyroid cell growth, however, are not mediated by changes in the TSH-induced cell cycle-related genes expression; both immediate early and progression genes expression is unaffected by the TGF beta treatment.