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Radioactive iodine: An isotope of the chemical element iodine that is radioactive. Radioactive iodine is used in diagnostic tests as well as in radiotherapy of an hyperactive thyroid gland (hyperthyroidism), most often due to Graves disease.

For hyperthyroidism, Radioactive iodine 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.

The majority of patients undergoing this treatment with radioactive iodine eventually become hypothyroid, which is easily treated using thyroid hormones (levothyroxine).

Radioactive iodine should not be used during pregnancy or breast feeding.

Radioactive iodine 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. Radioactive iodine is also known as radioiodine.

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