Re-evaluation of the thyroxine binding to human plasma lipoproteins using three techniques

Using gel filtration chromatography (GFC), we found that human VLDL, LDL and HDL transport approximate to0.03, 0.2 and 3% of plasma T-4. As T-4 could dissociate from carriers during GFC, we evaluated [I-125]T-4 transport to lipoproteins (Lp) in 50 normolipidemic and euthyroid subjects by GFC and 2 independent methods: zone electrophoresis (ZE) and radioimmunoprecipitation (RIP), At GFC, VLDL+LDL transported less T-4 (approximate to0.3%) than either ZE (approximate to1.3%) or RIP (approximate to1.5-2.0%), In contrast, GPC values for the HDL (approximate to3%) were close to the HIP values (approximate to3.5%), but lower than ZE (approximate to8.5%) because of partial co-migration with TBG, In conclusion, GFC underestimates T-4 binding to VLDL and LDL, which, indeed, is of the same magnitude as binding of some steroid hormones to CBG and SHBG, Hence, the anti-atherosclerotic effects of T-4 resulting from binding to the LDL should be greater than anticipated based on GPC data.