Background: thyroid transcription factor-1 (TTF-1) is known to play key roles in thyroid organogenesis, in thyroid cell proliferation and in the expression of genes involved in thyroid differentiated function. Objective: to understand why many human thyroid cancer cell lines keep producing TTF-1 despite the loss of differentiation. Methods: a chimeric protein acting as a functional antagonist of TTF-1 transcriptional activity was expressed conditionally in 8505C cells originating from an undifferentiated thyroid carcinoma. The consequences of the inhibition of TTF-1 activity in the cells expressing the antagonist on cell proliferation, mRNA and miRNA expression were analyzed. Results: we observed a growth arrest of 8505C thyroid cancer cells when the endogenous TTF-1 transcriptional activity was inhibited. It correlated with decreased levels of several mRNAs encoding positive effectors of cell proliferation like CDK1 and cyclinB1, and increased levels of various mRNAs encoding negative regulators of cell division like CDKN2B and DUSP6. By contrast, no significant change was observed in the miRNA population. Conclusion: the persistence of TTF-1 expression observed in most dedifferentiated human thyroid cancer cell lines is likely to be explained by the fact that TTF-1 activity is still required for proliferation of these tumor cells as demonstrated here in the 8505C cell line. Microarrays miRNA hybridizations were performed on 8505c cells expressing a TTF1 antagonist (Engr HD –Dox) versus non-expressing cells (Engr HD +Dox), on cells expressing the control protein (Engr HDm – Dox) versus non-expressing cells (Engr HDm + Dox), and on cell expressing the TTF1 antagonist (Engr HD– Dox) versus the cells expressing the control protein (Engr HDm– Dox). Hybridizations were replicated with dye swap.