We herein report a rare case of a 41-year-old woman with painless thyroiditis who was positive for TSH receptor-blocking (TBAbs) and receptor-stimulating autoantibodies (TSAbs) in the thyrotoxic phase. Her serum thyroid hormone levels were high, and TSH was undetectable. The low uptake of Tc-99m led to the diagnosis of painless thyroiditis. M22-TRAb, TBAb and TSAb were detectable in the thyrotoxic phase. Three months later, she became severely hypothyroid. M22-TRAb and TBAb were still strongly positive, although the TSAb levels had decreased to just above the reference range. In this case, TBAb led to hypothyroidism.