The thyroid is an endocrine gland located in the anterior part of the neck. By secreting thyroxine, triiodothyronine and calcitonin the thyroid affects almost every organ in the human body. Functional and structural disorders of the thyroid gland are very common. The basic classification of thyroid disorders divides them into the following entities: hypothyroidism, hyperthyroidism, thyroiditis, and thyroid tumors. Thyroid tumors are among the most common neoplasms in humans. Out of all the thyroid tumors, only three to five percent show malignant cell alteration. Thyroid tumors are divided into two main categories: differentiated and non-differentiated, which is extremely important since the classification into iodine-accumulating and iodine-non-accumulating cancers derives from it. Iodine-131 is a product of fission. Although its primary indication is in the therapy of hyperthyroidism, it is also used in the treatment of differentiated cancers. Regarding thyroid cancers, the indica-tions for iodine-131 therapy are: post-thyreoidectomy ablation of residual thyroid tissue, adjuvant therapy of subclinical residual disease, clinically clear residual disease therapy and extended disease therapy. To maximize the potential of radioiodine ablation (RAI) as a therapeutic modality, it is advisable to follow the guidelines of the American Thyroid Association (ATA) that divides patients into risk groups based on which a valid evaluation of the indications of the procedure is possible. The greatest benefit of iodine-131 therapy, combined with a surgical procedure, is demonstrated in patients with high risk cancer according to ATA classification. Valid and timely preparation of the patient for the procedure as well as adequate post-therapeutic follow-up are fundamental for successful iodine-131 therapy.