A woman patient in her 40s with differentiated thyroid cancer who had undergone total thyroidectomy underwent ¹³¹I radioiodine remnant ablation by administering 2.96 GBq of ¹³¹I at our hospital. ¹³¹I SPECT/CT images 10 days after administration showed abnormal accumulation of ¹³¹I in the left facet joint of the second lumbar vertebral bone. This abnormal uptake was considered to be due to bone metastasis of thyroid cancer, and surgical resection was performed. Histopathologically, no thyroid cancer cell was observed, but proliferations of small megakaryocytes were observed, which revealed myelodysplastic syndrome (MDS). To the best of our knowledge, there have been no reports of abnormal ¹³¹I accumulation in MDS.

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In two patients who received potassium iodide after discontinuing thiamazole and propylthiouracil for Graves' disease due to adverse reactions, ^99mTc thyroid scintigraphy performed prior to ¹³¹I administration after discontinuing potassium iodide and a 2-week low-iodine diet revealed RI accumulation in the thyroid gland was almost nonexistent. Blood free Thyroxine (fT4) were abnormally high and thyroid stimulating hormone (TSH) levels were abnormally low in both cases. Both patients were subsequently treated with 500 MBq of ¹³¹I. 24 hours later ¹³¹I scintigraphy showed ¹³¹I obvious accumulation in both thyroid lobes in both patients. Usually, the uptake functions of ¹³¹I are markedly higher than those of ^99mTc and thyroid uptake of ^99mTc is released within hours, while the ¹³¹I uptake increases over time. Discordances between ^99mTc and ¹³¹I uptake may reflect these differences.

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