Normal thyrocytes and thyroid cancer cells are characterized by possession of a sodium iodide symporter. Radioiodine administration is a unique and powerful means of treating differentiated thyroid cancer because of the ability of thyroid cancer cells to concentrate beta-emitting radiolabeled iodine. Several manipulations, such as iodine depletion and thyroid hormone-stimulating hormone elevation, are used to enhance uptake of radiolabeled iodine by tumor cells. Adjuvant radioiodine therapy, given to patients without evidence of residual disease, enhances the sensitivity of subsequent surveillance and may decrease recurrence rates and mortality. However, its exact role in the management of low-risk patients merits further investigation. In contrast, radioactive iodine therapy used in patients with residual or metastatic disease clearly improves outcomes. Several studies show decreased recurrence and mortality rates in patients treated with radioiodine compared with those not receiving radioactive iodine. Adverse events from radioiodine therapy include salivary gland dysfunction, bone marrow suppression, and reproductive disturbances. Side effects of radioiodine therapy are generally greater when higher activities of radioiodine are used and may be transient or permanent. Secondary malignancies also may occur after radioiodine therapy. These side effects must be weighed against potential benefits, especially when radioactive iodine is used as adjuvant therapy. Stimulation of the expression of the sodium iodide symporter, or its introduction de novo into nonthyroid cells, is promising in treating poorly differentiated thyroid cancer and nonthyroid malignancies, respectively.