サイロキシン結合蛋白を利用した甲状腺機能測定値におよぼす高遊離脂肪酸血症の影響
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概要
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After administration of the hypolipidemic agent 'dextran sulfate' to hyperlipidemic patients, the following data were acquired regardless of the thyroid function : the decrease of triglycerides, the increase of free fatty acids (FFA), the decrease of protein bound iodine, the increase of effective thyroxine ratio (ETR), the slight decrease of thyroid stimulating hormone, no significant change of thyroxine determined by radioimmunoassay (RIA-T<SUB>4</SUB>), the increase of T<SUB>4</SUB> determined by competitive protein-binding analysis (CPBA-T<SUB>4</SUB>) and the increase of resin triiodothyronine uptake (RT<SUB>3</SUB>U) determined by Scholer's method.<BR>Furthermore, it was recognized that the increase of CPBA-T<SUB>4</SUB> value was significantly correlative to the amount of increased FFA. Therefore, this paper was undertaken for the purpose of proving that FFA would interfere with the measurement system of commercial kits utilizing thyroxine binding proteins.<BR>It has recently been discovered that circulating FFA is mainly combined with albumin, but that oversupplied FFA is partially combined with lipoproteins or thyroxine binding globulin (TBG), and that T<SUB>4</SUB> is displaced by FFA from the TBG-binding site and becomes free.<BR>In hyper-free fatty acidemic serum, after the administration of dextran sulfate, it was naturally anticipated that the amount of free type of labelled T<SUB>3</SUB> must have increased in the system of saturation analysis, because FFA had already been tightly combined with the TBG site. But contrary to this expectation, the amount of separated free type of labelled T<SUB>3</SUB> decreased more than in normo-free fatty acidemic serum. This paradoxical finding was explained as the result of the surface activity-like interference of FFA with the absorption by resin strip. So it was proved that RT<SUB>3</SUB>U (Scholer's method) would be raised by oversupplied FFA.<BR>It was noticed in this study that when endogenous T<SUB>4</SUB> was extracted from serum in many commercial kits, the significant amount of FFA was also extracted by alcohol together with it. And because the extracted FFA also displaced labelled T<SUB>4</SUB> from the TBG site in the test vial, the free type of labelled T<SUB>4</SUB> increased more in the system of CPBA than when only endogenous T<SUB>4</SUB> competed with labelled T<SUB>4</SUB> from the TBG site. Thus the false elevation of CPBA-T<SUB>4</SUB> was calculated in hyper-free fatty acidemic serum. And the false elevation of ETR value in the same serum was proved as the result of the interference of FFA with the measurement system.<BR>Because the interference of FFA in RIA-T<SUB>4</SUB> was essentially negligible, the difference between CPBA-T<SUB>4</SUB> and RIA-T<SUB>4</SUB> values was significant in hyperthyroid patients with high levels of FFA.
- 日本内分泌学会の論文