Radioiodine-131 (I-131) is used in treatment of benign thyroid diseases with hyperthyroidism; toxic diffuse goiter, toxic nodular goiter and toxic multinodular goiter for long years. Treatment success depends on various factors. The most important factor affecting the success of treatment is the susceptibility to target tissue I-131. This review aims to give information about the physical and biological characteristics of I-131, advantages and disadvantages, indications and contraindications for I-131 therapy, patient preperation and administiration of I-131, dose selection, precautions on possible side effects, evaluation of treatment response and make an overview on the clinical studies about I-131 therapy.