Relationship between Serum Free Thyroxine, Total Thyroxine and Thyroxine-Binding Globulin Concentrations in Patients with Graves' Disease

Statistical analysis of relationship between free thyroxine (FT4) and thyroxine (T4) in patients with Graves' disease has revealed that serum FT4 values correlated well with serum T4 values at variable thyroid states before and during treatment with anti-thyroid drugs. However, the ratio of FT4 concentration to T4 concentration at hyperthyroid state was more than twice as large as the ratio at euthyroid and hypothyroid states. These findings are considered to denote the elevation of per cent of FT4 to T4 (% FT4) in thyrotoxicosis. Nevertheless, thyroxine-binding globulin (TBG) concentrations in hyperthyroid patients were slightly, but significantly lower by 7.3% than those in normal euthyroid volunteers. These results indicate the minimal contribution of the slight decrease in TBG concentrations to the marked elevation of %FT4 in patients with thyrotoxicosis.