Estimation of intracellular free triiodothyronine in man.

T3 RIA and measurements of dialyzable fraction T3 (DFT3) have been applied to human erythrocyte (RBC) cytoplasm in order to quantitate the intracellular free T3 (FT3) concentration in man. Mean intracellular T3, DFT3, and FT3 in RBCs from four normal subjects were, respectively, 12 ng/dl, 1.25%, and 0.15 mg/dl. These values were 10%, 450%, and 44% of those of T3, DFT3, and FT3 in plasma obtained from the same subjects. In RBCs from four hyperthyroid subjects, the mean cytosol DFT3 was slightly increased compared to euthyroid RBCs. Because intracellular T3 concentration was increased 8-fold, however, the cytosol FT3 was frankly elevated relative to euthyroid state cells. Thus, intracellular T3 and FT3 measurements have been shown to be feasible in the human RBC and confirm that increases in extracellular free T3 in fact accompanied by elevations of intracellular free hormone.