Preliminary studies on the measurement of the free thyroxine in the serum with Amerlex $FT_{4}$ RIA kit were investigated using a tracer as $^{125}I-T_{4}$ derivative which is not almostly bound to thyroxine binding globulin, etc. The results are followed: 1. Linearity was tested on standards at various concentrations, and reproducibility and accuracy was excellent. 2. The antibody specificity is also excellent, and standard calibration curve of total $T_{4}$ was similar that of adding the TBG inhibitor. 3. Each value of $T_{4}$ in serum (the normal group, the hypothyroidism patients the pregnant women and the TBG dificiency patients) was not significant. As mentioned above, this method is more simple and rapid, compared to the other method.