Studies on the clinical significance with Amerlex $FT_{4}$ RIA kit observing the determination of $FT_{4}$ 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. $FT_{4}$ value($1,55{pm}0.38ng/100ml$) of normal group was not accorded that of hyperthyroidism with Amerlex $FT_{4}$ RIA kit, and was higher than that of hypothyroidism. 2. $FT_{4}$ value was lower level in chronic-kidney disfunction syndrome whereas, it was normal in a cancer patient, a woman in pregnancy and a patient in TBG disfunction. 3. The value of this method is a good corelationship at that of equilibrium dialysis method. (r=0.931) 4. $FT_{4}$ value by this kit was linear relationship to those of the other kit (Gamma Coat and Liquisol), and the normal value of each methods was also similar. As mentioned above, this method is more simple and rapid, compared to the other method. Therefore, it was thought that this method is a very useful clinically.