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 approximately 0.03, 0.2 and 3% of plasma T4. As T4 could dissociate from carriers during GFC, we evaluated [125I]T4 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 T4 (approximately 0.3%) than either ZE (approximately 1.3%) or RIP (approximately 1.5-2.0%). In contrast, GFC values for the HDL (approximately 3%) were close to the RIP values (approximately 3.5%), but lower than ZE (approximately 8.5%) because of partial co-migration with TBG. In conclusion, GFC underestimates T4 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 T4 resulting from binding to the LDL should be greater than anticipated based on GFC data.