We present a 38-year-old man who underwent total thyroidectomy with radical right neck dissection due to papillary thyroid cancer was referred for 131I treatment. The patient was in subclinical hypothyroidism with remarkable stimulated Tg level after 4 weeks of l-thyroxine withdrawal before 131I treatment, which indicated hyperfunctioning metastasis. Posttherapeutic 131I whole-body scan combined with 131I SPECT/CT performed on day 3 after 131I administration revealed an elevated 131I uptake mass in cervicothoracic region. To our surprise, the mass was histologically confirmed to be a retrosternal goiter.
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