Thyroidal 99mTc uptake in the acute thyrotoxic phase of subacute thyroiditis (SAT) is always inhibited. However, a patient with SAT had signs in the right-side thyroid gland with transient thyrotoxicosis and slightly high 99mTc uptake levels in the right lobe, low 99mTc uptake in the left lobe, and normal overall uptake. Histological examination showed cellular destruction and granulomatous inflammatory changes in the right lobe, with marked interstitial fibrosis in the left lobe. The patient was thyrotrophin-receptor antibody (TRAb) positive. After a short course of prednisolone, SAT-like symptoms and signs improved. TRAb-positivity resolved spontaneously after 22 months, and TSH levels were slightly low for 22 months. Levels then kept normal in the following four years. In conclusion, high 99mTc uptake by the right lobe was due to the combined effects of TRAb and left thyroid gland fibrosis.