Thyroid disorders are common in individuals with Down syndrome (DS)2E Hyperthyroidism occurs much less frequently than hypothyroidism in this population, but is likely to be underestimated. We report a case of an institutionalized adult male with DS and hyperthyroidism. He was treated with radioactive iodine and, when reviewed 11 weeks later, was found to be markedly hypothyroid. We also review the literature on the three treatment options for hyperthyroidism in DS: surgery, medical treatment, and radiotherapy. We concluded that the place of radioiodine in the treatment of hyperthyroid patients with DS is yet to be defined.