An alternative to thyroidectomy or drug therapy, administration of the isotope iodine 131 (¹³¹I) or ¹²³I treats hyperthyroidism and is used adjunctively for thyroid cancer. After oral ingestion, ¹³¹I is rapidly absorbed and concentrated in the thyroid as if it were normal iodine. The result is acute radiation thyroiditis and
gradual thyroid atrophy that eventually reduces thyroid hormone levels.

When administered in the treatment of hyperthyroidism, ¹³¹I causes symptoms to subside after about 3 weeks and exerts its full effect after 3 months. A patient with acute hyperthyroidism may require ongoing drug therapy during this period. Similarly, a patient who also has cardiac disease should be given beta-adrenergic blockers to manage cardiovascular symptoms and antithyroid medications to decrease thyroid hormone production prior to receiving ¹³¹I to ablate the gland.

Although one ¹³¹I treatment usually suffices, a second or third treatment may be needed several months later if the patient has severe hyperthyroidism or an unusually large thyroid gland.

¹³¹I is the treatment of choice for nonpregnant adults who aren’t good candidates for spontaneous remission or who weren’t treated successfully with thyroid hormone antagonists. This relatively safe procedure exposes only the thyroid to radiation. However, it’s contraindicated during pregnancy because ¹³¹I does cross the placenta and can adversely affect the fetal thyroid gland. Despite the fact that no iatrogenic cancers have been documented in the more than 40 years ¹³¹I has been in use, this treatment is used cautiously in children and adolescents because of the potential for cancer or leukemia.