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Background: The efficacy of methimazole (MMI) combined with potassium iodine (KI) therapy as an initial treatment of Graves' disease (GD) is reported in an area of excessive iodine intake. However, transient aggravation of thyroid function after cessation of KI is often observed. Therefore, we evaluated the factors related with transient aggravation after cessation of KI in GD patients treated with MMI and KI therapy.

We herein report two cases of papillary thyroid carcinoma (PTC) after radioactive iodine therapy (RIT) for Graves' disease (GD). Case 1: A 25-year-old man visited to our clinic. He had undergone RIT for GD at 23 years old. His thyroid function was severe hypothyroidism because of poor compliance of taking levothyroxine (LT4), and thyroid nodule was detected by neck ultrasonography (US). After his first visit, his compliance of taking LT4 maintained poor, then his thyroid nodule grew up from 7 mm to 12 mm for 9 months. Fine needle aspiration biopsy (FNAB) was performed and the result of FNAB suggested PTC. Therefore, total thyroidectomy was done. Case 2: A 40-year-old woman was referred to our clinic for the treatment of PTC. She had also undergone RIT for GD at 34 years old. When she was 39 years old, a thyroid nodule was detected by neck US. Her compliance of taking LT4 was also poor, and the nodule grew up from 11 mm to 19 mm for 12 months. The result of FNAB was suggestive of PTC. Therefore, total thyroidectomy was done. In both two cases, thyroid nodular lesion could not be detected by neck US before RIT. In conclusion, we considered that this rapid growth of PTC in the irradiated thyroid gland may be induced by TSH stimulation, and that routine neck US should be continued after RIT for GD, even if there is no tumor in the thyroid before RIT.

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