In an attempt to evaluate the diagnostic significance of the serum thyroxine binding globulin(TBG) in various thyroid disease states, the authors measured serum T3 uptake, T3, total T4, free T4, TSH and TBG by radioimmunoassay technique, and calculated the free T4 index(FT4I) and T4/TBG ratio in 10 cases of normal subjects, 11 eases of hypothyroidism, 62 cases of euthyroidism and 37 cases of hyperthyroidism. The data were analysed in the aspects of diagnostic significance in each thyroid disease state, and the results were as follows; 1) In 10 cases of normal subjects, serum TBG was 17.4∼26.8ug/ml, FT4I was 5.1∼9.7, and T4/TBG ratio was 21.9∼49.9(Mean±S.D.). 2) 62 cases of euthyroidism with diffuse and nodular goiter, FT4I was 7.26±1.82, T4/TBG ratio was 31.47±10.42, and there were no significant difference from those of normal subjects(p〉0.5). 3) In 11 cases of hypothyroidism, the FT4I was 3.13±2.15, T4/TBG ratio was 11.3±5.31, significantly lower than normal controls(p〈0.01). 4) In 37 cases of hyperthyroidism, the FT4I was 30.0±12.01 T4/TBG ratio was 121.4±62.2, significantly higher than normal controls(p〈0.01). 5) There were significant correlations between the FT4I and T4/TBG ratio, total T4 and T4/TBG radio, in each thyroid function states. 6) The FT4I showed 100% of diagnostic value in hyperthyroidism, 89.2% in euthyroidism, and 80% in hypothyroidism group. The T4/TBG ratio showed 100% of diagnostic value in hyperthyroidism and hypothyroidism, and 80.6% in euthyroidism group. The above results suggest that T4/TBG ratio and FT4I showed same diagnostic value in hyperthyroidism group, but T4/TBG showed higher diagnostic significance than FT4I in hypothyroidism.