Re-evaluation of Thyroxine Binding and Free Thyroxine in Human Serum by Paper Electrophoresis and Equilibrium Dialysis, and a New Free Thyroxine Index1

Publisher Website
Google Scholar
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 calculated from the electrophoretic data obtained. The calculated values for Ktbg and Ktbpa were 2.2×1010m−1 and 1.9 ×108−1 using Kalb of 7 × 105m−1. The calculated value for free T4 in normal serum was 1.85 ng/ 100 ml, and the values calculated in sera from various thyroid states showed a close linear relationship to the measured values. Because changes in free TBPA capacity had little effect on free T4 concentration, a new free T4 index [PBI × T3resin uptake/(1−0.6 × T3resin uptake)] was derived using the estimated K values, a mean value for free TBPA capacity and the aforementioned relationship between free TBG capacity and TRU. The new index was shown to have a close linear relationship to the measured values for free T4 compared with other indices previously reported.
Keywords

This publication has 0 references indexed in Scilit: