Background: I-131 has been used in the therapy of well-differentiated thyroid cancer for over 50 years. Although the benefits and risks of I-131 remain issues of controversy and research, our understanding of them continues to improve. This review presents an overview of the benefits of I-131 therapy for ablation, adjuvant treatment, and treatment of locoregional and/or metastasis of well-differentiated thyroid cancer and considers the risks of complications of I-131 therapy.
Summary: The benefits of I-131 remnant ablation include:  facilitating the interpretation of subsequent serum thyroglobulin levels,  increasing the sensitivity of detection of locoregional and/or metastatic disease on subsequent follow-up radioactive iodine whole-body scans,  maximizing the therapeutic effect of subsequent treatments, and  allowing a postablation scan to help identify additional sites of disease that were not identified on the preablation scan or when a preablation scan was not performed. The potential benefits of I-131 adjuvant treatment include decreasing recurrence and disease-specific mortality for unknown microscopic, locoregional, and/or distant metastatic disease. The potential benefits of I-131 treatment of known locoregional and/or distant metastases are  decreasing recurrence, and  decreasing disease-specific mortality and/or palliation. The more significant risks and side effects involve organ systems including eye/nasolacrimal, salivary, pulmonary, gastrointestinal, hematopoietic, and gonads as well as secondary primary malignancies.
Conclusions: Although there are never-ending controversies regarding I-131 therapy in well-differentiated thyroid cancer, the benefits and risks are becoming better understood. This in turn helps the treating physician and patient in making decisions regarding therapy.