An Attempt to Compare the Affinity of Thyroxine-Binding-Globulin to Thyroxine in Normal and Hyperthyroid Sera

An approach to the comparison of the affinity of thyroxine-binding-globulin (TBG) in normal and hyperthyroid sera was described. The method was based on the measurement of free thyroxine fraction (FT4F) of sera at a fixed level of thyroxine (T4) and TBG-capacity. In normal and hyperthyroid sera, the concentration of T4 was measured with Murphy's method and TBG-capacity with reverse-flow paper electrophhoresis. The TBG-capacity was brought to a fixed level by diluting the sera to be tested with TBGdeficient serum. T4 was added to the sera at a final concentration of 50μg/100ml. FT4F in sera pretreated as described above was measured with a modification of Sterling's Mgprecipitation method employing barbital buffer, minimizing the effect of TBPA on FT4F.Thus, FT4F of normal and hyperthyroid sera was compared at essentially the same concentration of T4 and TBG-capacity. Therefore, the difference of FT4F must reflect the affinity of T4-TBG binding. With the method described, TBG-affinity of 6 hyperthyroid sera was compared with that of age-sex matched normal subjects. No significant difference of TBG-affinity was detected.