A Systematic Study of Variables Affecting Protein Binding of Thyroxine and Triiodothyronine in Serum

Using an ultrafiltration and resin dialysis method, the effect of pH, temperature, ionic medium, ionic strength, drugs, dilution, and total hormone concentrations on the free thyroxine (T4) and triiodothyronine (T3) fractions in serum is reported. It is shown that [1] the effect of pH, ionic medium, and tracer dose of T4 is stronger than previously realized; [2] endogenous dialysable competitors cause the so-called paradoxial dilution effect; [3] tris buffer, 0.16 mol/1, with pH 7.35 at 37 °C is a superior dilution medium; and [4] the ratio between T4 and T3 binding constants to thyroxine-binding globulin, thyroxine-binding prealbumin, and albumin are rougly 20, 10, and 6, respectively. This study suggests that the free T4 and T3 fractions in serum will vary in proportion to the various concentrations of total hormones or binding competitors in different states.