Iodine is a key element in the synthesis of thyroid hormones and the process of iodine transport by thyroid cells is active. Iodine deficiency, which is a known risk factor for thyroid cancer, may result from inadequate dietary iodine or from defects in iodine transportation. In contrast, a sufficient supply of iodine can induce cell-cycle arrest and apoptosis in thyroid cancer cells and may prevent the transformation of a differentiated thyroid cancer into a less differentiated or anaplastic form. However, the functions of iodine are complex, and the mechanisms of its action in thyroid cancer are unclear. This review focuses on the role of iodine in the carcinogenesis, proliferation, and apoptosis of thyroid cancers as well as on iodine transporters, most of which are impaired in thyroid cancer. Elucidating the roles of iodine may generate novel diagnostic and therapeutic approaches to thyroid cancer and improve current strategies for its treatment and prevention.