Objective: In the current study, we aimed to explore whether there is alteration between pre-and post-treatment micronucleus (MN) frequencies induced by internal and external ionizing radiation. Materials and Methods: The study enrolled a total of 67 patients including patients admitted to our hospital for treatment of hyperthyroidism (n = 17), scanning with low-dose I-131 (n = 15), and ablative therapy with high-dose I-131 (n = 15) at Department of Nuclear Medicine as well as patients with different diagnoses receiving external radiotherapy with various doses and durations at Department of Radiation Oncology (n = 20). Thirty-two patients who received radioactive iodine and returned for a follow-up visit at 1 month. Results: Considering both pre-and post-treatment MN frequencies of each group, lowest MN frequencies were detected for patients undergoing screening with low-dose I-131, and highest MN frequencies were found in radiotherapy patients. Comparison of pre-and post-treatment MN frequencies among hyperthyroidism, when pre-and post-treatment MN frequencies compared among hyperthyroidism, I-131 whole body scanning, ablation, and radiotherapy patient groups differences between MN frequencies were significant for each group (P < 0.05). Conclusion: Our study showed that MN analysis might be of value in determining chromosome damage that could potentially occur in patients exposed to internal and external radiation.