Epigenetic modification is one of the effective factors in tumorigenesis. Epigenetic processes, especially aberrant DNA methylation, play important role in thyroid cancer, and many tumor suppressor genes including PTEN, RASSF1A and TIMP3 are aberrantly methylated and silenced in thyroid cancer. Because of the specified pattern of DNA methylation in various tumor cells, it is suggested that these genes take part in a specific pathway. In addition to tumor suppressor genes, aberrant methylation in thyroid-specific genes such as those for sodium/iodide symporter and thyroid-stimulating hormone receptor is also common in thyroid tumorigenesis and progression. On the other hand, changes in these genes expression is one of the main cause for the failure of clinical radio-iodine treatment in subjects with thyroid cancer. Considering the importance of this issue, the present study intends to investigate the DNA methylation patterns of related genes in thyroid tumorogenesis.