In this paper, we report a 49-year-old female with subacute thyroiditis who had thyroidstimulating antibodies (TSAb) and thyroid-stimulation-blocking antibodies (TSBAb) in serum. Although she was in the thyrotoxic phase and TSH was suppressed in May, 1990, her radioactive iodine uptake (RAIU) was not suppressed (35.5%) and a thyroid scan disclosed a diffuse goiter with no defect. Serum assays revealed the presence of TSAb, but TSBAb were negative. In August, 1990, the right lobe became undetectable by thyroid scan when the RAIU was 20.7% with the TSH level remaining suppressed. At that time, TSAb were negative, while TSBAb were positive. When the RAIU was 31.1% in October, 1990, both thyroid lobes became visible and the TSH level was normalized. TSBAb became negative, and although TSAb reappeared it later became undetectable. These results indicate that the changes in the patient's thyroid scan and RAIU were attributable to the presence of TSAb.