The contribution of TFAM, TFB2M, and helicase Twinkle in thyroid-induced mtDNA biogenesis was assessed. The activation of TFAM and TFB2M expression is shown to be required for the induction of mtDNA biogenesis. The role of helicase Twinkle, the expression induction of which is also observed after triiodothyronine addition, remains unclear. The analysis of factors that activate TFAM and TFB2M expression showed that NRF-1 is the determinative regulator: