Why is radioactive iodine useful in diagnosing and treating certain thyroid conditions?

 It's used because the thyroid is the only organ in your body that absorbs iodine. Radioactive iodine scans show if the thyroid is overactive or under active (hyper and hypo) and it can help tell if certain types of thyroid cancer spread throughout the body. High doses of radioactive iodine basically cause the thyroid to kill itself by up taking massive amounts of iodine, and it also minimizes the radiation exposure to the rest of your body.

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