Which radioisotope of Iodine is used in the treatment of hypehyroidism?

Iodine 131 when taken up by thyroid gland result in destruction of the trapping cell and cells in close proximity. Because thyroid gland has extremely high affinity for iodine compared to other tissues, radioactive iodine results in selective ablation of thyroid tissue. Ref: Diseases of the Thyroid in Childhood and Adolescence By Gerasimos E. Krassas, Volume 11, Page 172; Encyclopedia of Diagnostic Imaging By Albe L. Bae, Volume 1, Page 452; Iodine an Overview, Page 18.