A radioactive isotope of iodine. (An isotope is an alternate version of a chemical element that has a different atomic mass). Radioiodine can be used in diagnostic tests as well as in radiotherapy of the thyroid.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. The majority of patients undergoing this treatment eventually become hypothyroid, which is easily treated using thyroid hormones (levothyroxine). Radioiodine is contraindicated during pregnancy and breast feeding.