Further experience with free thyroxine assays with particular reference to pregnancy

The Amerlex FT4 radioimmunoassay has been assessed. It is simple, quick, and precise. The assay is as good as the FTI or the Immo Phase FT4 assay in hyperthyroid, hypothyroid, or elderly subjects and in women on oral contraceptives. In acute illness and pregnancy, however, over half the values lie below the euthyroid reference range. Further work on blood samples obtained from pregnant subjects demonstrated that while FT4 values given by the Immo Phase and Liquisol kits remain constant, those given by the GammaCoat, Amerlex, and Lisophase kits show a significant decrease with gestation. What the 'true' FT4 is in acute illness and in pregnancy remains an open question.