In an attempt to evaluate the diagnostic significance of the serum thyroid hormones in various thyroid function states, the author measured serum $T_3$ uptake, serum $T_3$, serum $T_4$, serum free $T_4$ and free $T_3$ index in 27 cases of normal subjects, 11 cases of hypothyroidism, 152 cases of euthyroidism and 81 cases of hyperthyroidism by the radioimmunoassay method. The results were as follows: 1. The ranges of serum thyroid hormones in normal subjects were, serum $T_3$ uptake; $27.4{sim}42.1%$, serum $T_3;;93{sim}245ng/dl$, serum $T_4;;4.08{sim}12.9ug/dl$ and serum free $T_4;;0.57{sim}1.53ng/dl(M{pm}2S.D.)$. 2. Free $T_4$ index and serum $T_4$ show relatively high diagnostic value in euthyroidism group, and serum $T_3;and;T_4$ in hypothyroidism group, while serum $T_3$, free $T_4;and;T_4$ show relatively high diagnostic value in hyperthyroidism group. 3. There were significant correlation between free $T_4$ index and serum $T_4$(r=0.68), and between free $T_4$ index and serum free $T_4$(r=0.67) in hyperthyroidism group.