A 55-year female patient presented with subacute thyroiditis (SAT) with a unique dynamic evolution, along with changes in the level of antithyroglobulin antibody, which has been rarely reported. Her thyrotoxicosis gradually worsened over the next three months. Severe hypothyroidism then rapidly developed and did not resolve. For the whole disease course, antithyroglobulin antibody levels were significantly increased, indicating dynamic changes in thyroid function. It has been suggested that the duration of thyrotoxicosis in SAT is highly variable, which is probably related to an underlying autoimmune mechanism. It is therefore, necessary to rule out other causes of thyroiditis.