〔Abstract〕 Objective To study the effects of different iodine intake on thyroid function and the methylation of 5′-CpG island in the promoter region of sodium iodine transporter(NIS) gene in rats, and to explore whether iodine affects the expression level of NIS through regulating the methylation of NIS gene. Methods Forty female SD rats were randomly divided into five groups(8 rats in each group). The rats in NI group(control) were fed with normal content of iodine, and the rats in LI group, 5 HI group,10 HI Group and 20 HI group were fed with different amounts of potassium iodide. These rats were sacrificed 3 months after the feeding and all serum samples were tested for free triiodothyronine(FT3),free thyroxine(FT4) and thyroid stimulating hormone(TSH) by using chemiluminescent detection technology. The expression of NIS protein in the rats' thyroid tissues was determined by immunohistochemical staining. The status of 5′-CpG island promoter methylation of NIS gene in the rats' thyroid tissues wasdetected by bisulfite sequencing PCR(BSP). Results ① Compared with the iodine-adapted group, the serum FT3 concentration of the low-iodine group and the high-iodine group decreased, and the difference was significant(P<0.05). Compared with the iodine-adapted group, the serum FT4 in the low-iodine group and the high-iodine group slightly increased, but the difference was not statistically significant(P>0.05). There was no significant difference in TSH level between groups(P>0.05).② The expression level of NIS in thyroid tissues of the LI group was higher than that of the NI group, with statistically significant difference(P<0.05), while that of the 5 HI group was lower than that of the NI group, with no statistically significant difference(P>0.05), while those of the 10 HI group and the 20 HI group were lower than that of the NI group(P<0.01);③ There was no significant difference in CpG1 methylation ratio and CpG2 methylation ratio between the groups(P>0.05). Further study on the sequence of-420 to-110 nucleotides in the NIS promoter region(CpG3) showed that compared with the normal iodine group, the methylation ratio of CpG3 in the low-iodine group and the high-iodine group decreased, but the difference was not statistically significant(P>0.05). Conclusion Iodine overdose or iodine deficiency can cause iodine nutritional disorders, but it does not affect the methylation status of the 5′-CpG island promoter methylation of sodium iodide symporter gene in rats. Iodine may affect the expression level of NIS through other ways.