This therapy used for thyroid cancer. Your thyroid gland absorbs nearly all of the iodine in your body. When radioactive iodine (RAI), is taken into the body in liquid or capsule form, it concentrates in thyroid cells. The radiation can destroy the thyroid gland and any other thyroid cells.
When a thyroid gland is overactive, it produces too much of these hormones, accelerating the metabolism. Radioactive iodine (I-131), an isotope of iodine that emits radiation, is used for medical purposes. When a small dose of I-131 is swallowed, it is absorbed into the bloodstream in the gastrointestinal (GI) tract and concentrated from the blood by the thyroid gland, where it begins destroying the gland's cells.  Radioactive iodine (radioiodine) therapy for thyroid cancer. Your thyroid gland absorbs nearly all of the iodine in your body. When radioactive iodine (RAI), also known as I-131, is taken into the body in liquid or capsule form, it concentrates in thyroid cells. The radiation can destroy the thyroid gland and any other thyroid cells (including cancer cells) that take up iodine, with little effect on the rest of your body The goal of treatment is to lower the amount of thyroid hormones. Treatment options include. 1  Radioactive iodine (RAI). 2  Also called radioiodine, RAI is a common and long-used treatment for hyperthyroidism. 3  Surgery. 4  This removes the thyroid gland (called thyroidectomy). 5  Antithyroid drugs. 6  These medicines are given for months or even years.  Radioactive iodine (commonly called radioiodine) is a form of iodine chemically identical to nonradioactive iodine. Therefore, the thyroid gland, which takes up iodine to make thyroid hormone, cannot distinguish between the two. Radioactive iodine treatment of hyperthyroidism makes use of the thyroid gland's natural need for iodine to make thyroid hormone. The thyroid is the only part of the body that collects and retains iodine. In hyperthyroidism, the thyroid cells are over-stimulated and make larger amounts of thyroid hormone You may receive radioactive iodine (also known as radioiodine, I-131 or RAI) some weeks after surgery to eliminate (ablate) any remaining papillary or follicular thyroid cancer cells or normal thyroid tissue that the surgeon could not remove.  Guide. Radioactive iodine is a medicine that you take one time. After you swallow it, it is taken up by your thyroid gland. Depending on the dosage used, the radioactivity in the iodine destroys most or all of the tissue in your thyroid gland, but it does not harm any other parts of your body.  Radioactive iodine, given in a capsule or liquid form, is absorbed and concentrated by the thyroid gland. The treatment destroys thyroid tissue but does not harm other tissue in the body. See a picture of the thyroid gland. While radiation can cause thyroid cancer, treatment of hyperthyroidism with radioactive iodine does not increase your chances of getting thyroid cancer. Iodine is taken up by the thyroid gland and most thyroid cancers. When the cells take up iodine, they also take up radioiodine or iodine-131, which is a radioactive form of iodine. Iodine that is radioactive can be used to find and treat thyroid cancer. Radioactive iodine is one of the oldest and best forms of cancer treatment for any thyroid cancer that will take up radioiodine. It destroys cancer cells. Radioiodine (sodium I-131) is a form of radiation therapy that has been used for many years to treat thyroid conditions. It is safe and effective but requires you to observe certain precautions to decrease the small amount of radiation that other people may receive from your body and bodily fluids. 
http://www.radiologyinfo.org/en/info.cfm?PG=radioiodine http://www.cancer.org/cancer/thyroidcancer/detailedguide/thyroid-cancer-treating-radioactive-iodine http://www.hormone.org/questions-and-answers/2012/radioactive-iodine-treatment-for-hyperthyroidism http://www.thyroid.ca/e12a.php http://www.thyca.org/pap-fol/rai/ http://www.webmd.com/women/radioactive-iodine-for-hyperthyroidism http://www.webmd.com/women/radioactive-iodine http://www.uwhealth.org/surgery/radioactive-iodine/10519 http://www.snmmi.org/AboutSNMMI/Content.aspx?ItemNumber=5609
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