Inhibitors of histone deacetylases (HDACs) activate the sodium iodide symporter (NIS) expression in thyroid tumor cells. In this study, mechanisms accounting for these effects were investigated. Various human thyroid tumor cell lines (ARO, BCPAP, FRO, TPC-1) were treated with the HDAC inhibitors Na butyrate (NaB) and Tricostatin A (TSA) and the effects on the expression of NIS and several thyroid-specific transcription factors together with the activity of NIS promoter were evaluated. TSA and NaB increased NIS mRNA levels in all cell lines. Among thyroid-specific transcription factors, only expression of PAX8 in ARO cells was increased. Down-regulation of TTF-1 expression was observed in BCPAP and TPC-1 cell lines. TTF-2 mRNA was reduced in FRO, BCPAP and TPC-1 cells. Histone acetylation had no significant effects on HEX expression. Altogether, these data indicate that the increase of NIS expression is not mediated by modification of expression of thyroid-specific transcription factors. Accordingly, in transfection experiments performed in HeLa cell line (which does not express thyroid-specific transcription factors), treatment with TSA and NaB increased NIS promoter activity. Stimulation of NIS promoter activity was also obtained by overexpressing histone acetylating proteins pCAF and p300 in HeLa cells. Conversely, overexpression of the Histone deacetylase 1 enzyme (HDAC1) inhibited basal activity of NIS promoter. Effects of TSA and NaB on NIS expression were also evaluated in non-thyroid cell lines MCF-7, Hep-G2 and SAOS-2. In all cell lines TSA and NaB greatly increased NIS mRNA levels. Thus, control of NIS expression by inhibition of histone deacetylase appears not to be mediated by cell-specific mechanisms, suggesting it as a potential strategy to induce radioiodine sensitivity in different human tumors.