Hyperfunctioning thyroid carcinoma is very rare. Hence, radionuclide imaging of thyroid hot nodules usually suggests a benign tumor, and less than 4% of cases have been reported as malignant. We would like to present a case of a hyperfunctioning papillary thyroid carcinoma that was initially treated with radioactive iodine. A 58-year-old woman was referred to our hospital for palpable thyroid nodule and a 5-kg weight loss within 6 months. Thyroid function test revealed thyrotoxicosis, and thyroid autoantibodies were absent. Tc thyroid scintigraphy showed a 2 × 2 cm-sized hyperactive hot nodule at the left lobe. Despite radioactive iodine treatment with a dose of 10 mCi 131 I, thyroid function did not improve. Fine needle aspiration revealed papillary thyroid cancer. The patient underwent total thyroidectomy. Although clinical features and thyroid scans suggest a benign nodule, the possibility of malignancy should not be ruled out. Malignant thyroid hot nodules are rare; however, its possibility should be taken into account. Therefore, we suggest that ruling out malignancy by existing diagnostic guidelines can misdiagnose even a typical case with benign features. As thyroid nodule detection is getting sensitive and accurate, we present this case to discuss whether additional diagnostic approaches would be necessary for thyroid nodules. (Endocrinol Metab 27:59-62, 2012)

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A 30-year-old man was admitted to our hospital because of fatigue, palpitation and severe weakness of both legs. The admission laboratory findings revealed thyrotoxicosis, and 131 I thyroid scintigraphic imaging revealed a low radioactive iodine uptake. He was treated for painless thyroiditis for about 4 months. However, thyrotoxic state had continued and radioactive iodine uptake was markedly increased in the follow up scan. Painless thyroiditis often relapses, but rarely develops into Graves' disease. This is a rare case in which painless thyroiditis was followed by Graves' disease. (Endocrinol Metab 27:147-150, 2012)

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