Most published radioimmunoassay methods to determine triiodothyronine (T3) concentrations in serum depend on addition prior to the assay of chemical compounds designed to release T3 bound to thyroid hormone binding proteins (TBP). Instead, the present method has employed denaturation of TBP by simple extraction with 95% ethanol for this purpose, followed by radioimmunoassay. Serums from 127 seemingly healthy subjects aged 5 to 93 showed a statistically significant, progressive decrease for both sexes in T3 concentration with advancing age, amounting to 5.1 ng/dl per decade. Mean T3 concentrations followed the regression line reasonably closely. However, the mean, 108.6 ng/dl, in the 15 to 19 year group, was probably skewed significantly. Mean concentrations reached a high, 140.8 ng/dl in the 5 to 9 yr group, low, 92.2 ng/dl in the 80 to 93 yr group, with the overall mean 119 ± 6.3 (SE) ng/dl. The normal mean and the means for hypothyroid, 46 ng/dl (range < 20–92), and hyperthyroid 353 ng/dl (range 176–556), serums, correspond well with the results reported by most groups working with radioimmunoassay.