Serum total thyroxine (T4) was measured in clinically euthyroid and clinically hyperthyroid subjects, using the method of competitive protein binding. In 26 euthyroid subjects the mean +/- 1SD was 7.7 +/- 1.8 micrograms/dl and in 17 clinically hyperthyroid subjects, the mean +/- 1SD was 18.1 +/- 3.1 micrograms/dl. This difference is statistically significant. Serum % 125(1) -- T3 binding was measured using the principle of competition for 125(1) -- T3 between the unknown serum and the resin in the assay kit. In 18 clinically euthyroid subjects, the mean +/- 1SD was 112.6 +/- 4.4 % while in 14 clinically hyperthyroid subjects the % 125(1) -- T3 binding had a mean +/- 1SD of 71.4 +/- 11%. The difference is statistically significant. Serum total triiodothyronine was measured by radioimmunoassay. In 31 clinically euthyroid subjects the mean +/- 1SD was 146 +/- 19.8 ng/dl while in 19 clinically hyperthyroid sugjects the mean +/- 1SD was 390 +/- 111 ng/dl. This difference is statistically significant. There was a negative correlation which was not statistically significant between the serum T4 and % T3 binding. The measurements of these thyroid function indices were precise.