The thyroid transcription factor TTF-S is a forkhead-containing protein involved in thyroid-specific gene expression and necessary for thyroid morphogenesis. In this paper, we demonstrate that TTF-2 is able to inhibit the activity of the thyroid-specific transcription factors TTF-1 and Pax-8 only on certain promoters. We identified the minimal protein domain responsible for repressor activity, which behaves as an independent functional domain, and me show that repression by TTF-g is DNA-binding independent. We suggest that TTF-2 is able to interfere with a specific cofactor required for TTF-1 and Pax-8 activity.