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Abstract: 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., Competing Interests: Conflicts of interest and sources of funding: This work was supported by the National Natural Science Foundation of China (grant number 81771865). The authors have no conflicts of interest to declare., (Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.)

A 63-year-old man with papillary thyroid carcinoma (PTC) and bone metastases was treated with 131I residual thyroid ablation (RTA) and 2 repeated post-ablation 131I therapies after total thyroidectomy. 131I whole-body scans (WBS) demonstrated sustained and diffuse 131I avid bone metastases, accompanied by persistent very low thyroglobulin (Tg) levels and negative antithyroglobulin antibody (TgAb) in post-ablation 131I therapies in hypothyroid state after levothyroxine withdrawal. The spread of bone metastases were found on the last therapeutical 131I WBS. The benefit of 131I therapy after levothyroxine withdrawal should be weighed even if the metastases are 131I avid in poorly differentiated thyroid carcinoma.