A new method for the quantitative determination of free triiodothyronine (
fT
3) and an accurate method for the quantitative determination of free thyroxine (
fT
4) in human plasma were developed.
The percentages of T
3 and T
4 in plasma were studied by thin-layer chromato-graphy. An elutable fraction (EF) of T
3, ratio of free
125I-T
3 to protein-bound
125I-T
3, was obtained using Sephadex-microcolumn chromatography. EF of T
4 was obtained by the same method.
Mean normal total T
3 (
tT
3) and total T
4 (
tT
4), calculated from plasma butanol-extractable iodine (BET) and molar percentages of T
3 and T
4, were 0. 63 and 5. 59μg per 100ml plasma, respectively.
Mean normal
fT
3 and
fT
4, calculated from
tT
3,
tT
4 and EF values of T
3 and T
4, were 2.43 and 2.21mμg per 100ml plasma, respectively.
The
fT
3 values can be obtained by the present method more easily and speedily than by the method of Nauman
et al. The present
fT
4 values, in consideration of BET and the percentage of T
4 in plasma, represent a more accurate one than the earlier data by Lee
et al.
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