Free thyroxine (FT4) is the analyte of choice in early gestation
B Dando1, R Taylor1, R Henley1, AB Parkes2, R John1 & JH Lazarus2
Thyroid physiology in early pregnancy is characterised by an increase of thyroxine binding globulin. This has been taken into account when using total T4 (TT4) if a T3 uptake test (TUp) is used to derive the FTI. The American Thyroid Association Guidelines (1990) recommend ceasing the use of uptake tests, although in the USA they are still widely used. The aim of this study is to compare the TT4 and TUp to the FT4 in early gestation. TT4, TUp, FT4 and TSH were measured by ADVIA Centaur (Bayer) in 980 samples from 1116 weeks gestation.
Results indicated a curvilinear relationship between TT4 and TUp for TT4 concentrations between 0.1 and 15.0 μg/l. A plot of FTI vs FT4 showed 24 samples with low FTI but normal FT4 (up to 18 pmol/l). TSH plotted against FT4 showed a closer relationship than TSH vs FTI.
It is concluded that thyroid binding protein increase in gestation is not a single linear function (especially at extremes of thyroid function). Therefore, the FTI is not always compensated for by the TUp test. FT4 is therefore the analyte of choice for determining thyroid function in early pregnancy.