Use of radioactive iodine is an essential adjuvant treatment strategy after thyroidectomy in patients with differentiated thyroid carcinoma. Although generally safe, radioiodine therapy has some potential side effects, classified as early and late complications, which we have reviewed in this paper. Early complications include gastrointestinal symptoms, radiation thyroiditis, sialadenitis/xerostomia, bone marrow suppression, gonadal damage, dry eye, and nasolacrimal duct obstruction. The late complications include secondary cancers, pulmonary fibrosis, permanent bone marrow suppression, and genetic effects. As 131I is an efficacious form of treatment that can significantly decrease the rate of mortality, recurrence, and metastasis, and as the side effects are often minor and well tolerated, radioiodine therapy remains the principal mode of treatment for patients with differentiated thyroid carcinoma.