A case of TSH receptor antibody-positive hyperthyroidism with functioning metastases of thyroid carcinoma

The presence of TSH receptor antibody (TRAb) is rarely responsible for hyperthyroidism due to metastatic lesions of thyroid carcinoma. A 70-year-old woman was incidentally found to be thyrotoxic around the time that external irradiation was performed for multiple bone metastases 9 years after subtotal thyroidectomy for follicular carcinoma. Hyperthyroidism persisted after oral administration of thiamazole. Relevant laboratory data were as follows: FT4 9.6 ng/L, FT3 7.3 ng/L, TSH <0.19 mU/L, TBII 70, TSAb 735, and Tg 32,000 μg/L. 131I-total body scan showed 131I accumulation in the occipital bone, cervical vertebra, thoracic vertebra, ilium, and residual thyroid gland. Since the ilium uptake (11.6) was markedly higher compared to the residual thyroid gland uptake (0.14), four subsequent 131I therapies were performed. The patient became hypothyroid, and TBII became negative. TSAb became negative after the first 131I-therapy but has increased again to 204 at present. Tg was 1,962 μg/L despite high TSH levels. 131I accumulation in the residual thyroid, cervical vertebra, and thoracic vertebra disappeared. Also 131I accumulation in the ilium has gradually decreased, but the image in the occipital bone has become markedly distinctive. This is a rare case characterized by TRAb-positive hyperthyroidism, by T3-predominant thyrotoxicosis, and by stronger accumulation of 131I in the metastatic tumor than in the residual thyroid gland. Thus, the response to TRAb and 131I-therapy is different among metastatic thyroid tissues.