The aim of this study was to comparatively investigate the effects of 5-azacytidine-C (5-Aza), trichostatin-A (TSA), and all-trans retinoic acid (ATRA) on mRNA expressions of Na/I symporter (NIS), thyroglobulin (Tg), thyroid peroxidase (TPO), and thyroid stimulating hormone receptor (TSH-R), and radioiodine (RAI) uptake in cancer (B-CPAP) and normal (Nthy-ori 3-1) thyroid cell lines. Cell lines were treated with 10 ng/mL of TSA, 5 mu M of 5-Aza, and 1 mu M of ATRA, according to the MTT (methyl-thiazol-tetra-zolium) test results. Additionally, recombinant thyroid stimulating hormone (rTSH) was also applied, with a selected dose of 100 ng/mL. Following the treatment, NIS, Tg, TPO, and TSH-R mRNA levels were detected by real-time-polymerase chain reaction (RT-PCR) and RAI uptakes were measured by using a well counter as the counts/cell number. 5-Aza increased TSH-R mRNA expression in both of the cell lines and decreased TPO, NIS, and Tg mRNA levels in the cancer cell line. In the normal thyroid cell line, 5-Aza increased TPO mRNA levels 2-fold and made no differences in NIS and Tg mRNA levels. TSA treatment repressed NIS and Tg mRNA levels, and made no differences on other thyroid specific genes investigated in the cancer cell line. In the normal thyroid cell line, TSA increased TSH-R mRNA levels in 72 hours and created no important differences in other genes. ATRA repressed the TSH-R mRAA levels in the normal thyroid cell line and increased the TPO and Tg mRNA levels slightly in both cell lines. Furthermore, in short-term treatment, ATRA repressed NIS gene expression slightly, but in the long term, this repression turned to basal levels. 5-Aza, TSA, and A IRA did not make any differences in RAI uptake in the cancer cell line, but rTSH increased RAI uptake significantly. In the normal thyroid cell line, TSA and ATRA decreased RAI uptake (to 1/10 and 1/2, respectively), but 5-Aza and rTSH increased RAI uptake significantly (2- and 4-fold, respectively). We have shown an increase in TSH-R gene expression and radioiodine uptake with 5-Aza. Further in vitro and in vivo studies are needed to support our findings and the potential clinical use of this agent.