We report a case of painless thyroiditis detected during the first trimester of pregnancy. A 29-year-old Japanese woman was hospitalized because of thyrotoxicosis and she was confirmed to be pregnant. The gestational age was 4 weeks. Blood examinations revealed negative TSH receptor antibodies, however, we started potassium iodide because we were unable to rule out Graves' disease. Thyroid hormone levels were normalized in 3 weeks and remained low even after discontinuation of medication. She received replacement therapy with levothyroxine sodium hydrate till 3 months after delivery. Painless thyroiditis can be one of the differential diagnoses of thyrotoxicosis in a very early stage of pregnancy.