This study examined the relationship between oxidative DNA damage assessed by urinary 8-hydroxy-2′-deoxyguanosine (8-oxoG) and polymorphisms in two key base excision repair enzymes, hOGG1 (Ser326Cys) and APE-1 (Asp148Glu), in patients with differentiated thyroid cancer (DTC) treated with high-dose radioactive iodine (RAI). This evidence concerns the gene APEX1 and differentiated thyroid carcinoma.