Radioiodine (131I) has been used to ablate thyroid tissue not removed by surgery or to treat differentiated thyroid cancer that has metastasized to other parts of the body for the past 80 years. However, the Na+/I- symporter (NIS), which mediates active iodide uptake into thyroid follicular cells, is also expressed in several non-thyroidal tissues. This NIS expression permits 131I accumulation and radiation damage in these non-target tissues, which accounts for the adverse effects of radioiodine therapy. We will review the data regarding the expression, function, and regulation of NIS in non-thyroidal tissues. We will explain the seemingly paradoxical adverse effects induced by 131I: the self-limited gastrointestinal adverse effects in contrast to the permanent salivary dysfunction that is seen after 131I therapy. We propose that prospective studies are needed to uncover the time-course of pathological processes underlying development and progression or ultimate resolution of 131I-induced salivary ductal obstruction and nasolacrimal duct obstruction. Finally, preventive measures and early therapeutic interventions that can be applied potentially to eliminate or alleviate long-term radioiodine adverse effects will be discussed.