Quantitative Determination of Total and Free Triiodothyronine and Thyroxine

Abstract

A new method for the quantitative determination of free triiodothyronine (_fT₃) and an accurate method for the quantitative determination of free thyroxine (_fT₄) in human plasma were developed.
The percentages of T₃ and T₄ in plasma were studied by thin-layer chromato-graphy. An elutable fraction (EF) of T₃, ratio of free ¹²⁵I-T₃ to protein-bound ¹²⁵I-T₃, was obtained using Sephadex-microcolumn chromatography. EF of T₄ was obtained by the same method.
Mean normal total T₃ (_tT₃) and total T₄ (_tT₄), calculated from plasma butanol-extractable iodine (BET) and molar percentages of T₃ and T₄, were 0. 63 and 5. 59μg per 100ml plasma, respectively.
Mean normal _fT₃ and _fT₄, calculated from _tT₃, _tT₄ and EF values of T₃ and T₄, were 2.43 and 2.21mμg per 100ml plasma, respectively.
The _fT₃ values can be obtained by the present method more easily and speedily than by the method of Nauman et al. The present _fT₄ values, in consideration of BET and the percentage of T₄ in plasma, represent a more accurate one than the earlier data by Lee et al.