Radioiodine: An isotope of the chemical element iodine that is radioactive. Radioiodine is used in diagnostic tests as well as in radiotherapy of an hyperactive thyroid gland (hyperthyroidism), most often due to Graves' disease.
For hyperthyroidism, radioiodine is administered in capsule form on a one- time basis. It directly radiates thyroid tissues, thereby destroying them. It takes 8-12 weeks for the thyroid to become euthyroid (normal) after treatment.
Radioiodine is the preferred initial therapy for the hyperthyroidism of Graves disease in North America while antithyroid drugs are used as the first line of treatment in most of the rest of the world.