Abstract
The physico-chemical interactions between thyroxine (T4) and thyroxine-binding globulin (TBG) of low binding capacity were investigated by radioimmunoassay, equilibrium dialysis, and reverse flow electrophoresis. Knowledge of total and free T4 (FT4) concentrations in serum, and of the total binding capacity of the protein carrier (TTBG), allowed the determination of the association constant ruling these interactions (Ktbg). Correlation between T4 and FT4 varies with TTBG, shifting the normal range for T4. The level of FT4 in serum is a function of the fractional saturation of TBG by endogenous hormones, which depends on T4 and TTBG. Data on T4, TTBG, and Ktbg were integrated into the general equation of the law of mass action and the results showed a very significant linear correlation with the values of FT4 measured by equilibrium dialysis. It is concluded that the misleading results of serum T4 measurements and of the free T4 index, obtained in euthyroid individuals with low TTBG, cannot be ascribed to a reduction of the intrinsic sensitivity of the assay due to oversaturation of TBG by endogenous T4, as previously postulated by others, but to a shift of their normal ranges produced by abnormal variations of TTBG. These results stress the need for data regarding TTBG for the proper interpretation of T4, and for the calculation of the fractional saturation of TBG and the FT4 concentration in serum. We have solved this problem by using an empirical equation relating TTBG to T4 and triiodothyronine (T3) uptake, which was previously derived by other workers.