Re-evaluation of thyroxine binding and free thyroxine in human serum by paper electrophoresis and equilibrium dialysis, and a new free thyroxine index.

Abstract

ABSTRACT Thyroxine (T4) binding by 3 protein carriers (TBG, TBPA and albumin) was assessed in sera from various thyroid states using reverse flow paper electrophoresis, with which in normal serum 76% of endogenous T4 was bound to TBG, 15 % to TBPA and 9 % to albumin despite maximal TBG and TBPA capacities, consistent with previous reports. The percentage of endogenous T4 bound to TBG was increased to 87% in hypothyroidism and 88% in pregnancy, while it was decreased to 42 % in hyperthyroidism and 62 % in patients treated with anabolic steroids. Our results indicate that, in the above conditions, the role of TBG in T4 transport is even more important than has been previously recognized. On the basis of the newly estimated distribution of endogenous T4, the following relationship was established between free TBG capacity and triiodothyronine (T3) resin uptake (TRU): free TBG capacity (μg/100 ml) =5.02/ TRU(%/100)–4.98. Further, association constants of TBG and TBPA for T4, and free T4 concentrations were ca...