Abstract: Thyroid function was tested in patients with hyperthyroidism (40 cases), subacute thyroiditis (12) and painless thyroiditis (21). The serum free triiodothyronine/free thyroxine ratio value (FT_3/FT_4)was>0.24 pmol/pmol in hyperthyroidism but<0.24 in 11/12 patients with subacute thyroiditis and 20/21 patients with painless thyroiditis. Therefore, like RAIU of thyroid, the serum FT_3/FT_4 ratio value is useful for differentiating painless thyroiditis from the hyperthyroidism of Graves' disease.