In most cancers peripheral blood lymphocytes exhibit DNA damage. In the case of thyroid cancer the micronucleus (MN) assay has been used to assess DNA damage before and after exposure to iodine-131 (¹³¹I). The aim of our study was to use this method to assess DNA damage in peripheral blood lymphocytes of thyroid cancer patients and search for its relationship with metastasis as well as ¹³¹I exposure. A significant increase in micronuclei frequency was observed in peripheral blood lymphocytes of 54 thyroid cancer patients in comparison to 38 controls (p = 0.000). Further analysis revealed significant elevation in micronuclei index from 48.5 MN/1000 BN cells (range: 25.1–111.2, n = 25) in patients without metastasis to 68.1 MN/1000 BN cells (range: 26.2–135.5, n = 29, p = 0.001) in group of patients with metastasis to one or more sites. There was no clear correlation between the micronuclei frequency and the therapeutic ¹³¹I dose ranging from 0.41 to 31.5 GBq with the exposure interval of <1 to 126 months. In addition, age and sex did not show any influence on micronuclei frequency in either patients or control population. These findings are indicative of increased basal DNA damage in thyroid cancer patients before treatment. Radioiodine treatment did not increase DNA damage measured by the micronuclei frequency for the interval between the last radioiodine dose administered and analysis of blood sample. However a significant increase of peripheral blood lymphocytes micro- nuclei was observed in thyroid cancer patients with metastasis.