The present study was undertaken to examine serum concentration of free $T_3,;T_4$, in various functional states of the thyroid by Amerlex RIA kit method, which uses unidentified $T_3,;T_4-labelled$ analogues, said to be unreactive with $T_3,;T_4-binding$ proteins in serum, together with an antibody that binds both analogue and $T_3,;T_4$. The test method has been compared with free $T_4$ index, which was product of total serum thyroxine and $T_3$ resin uptake ratio. Free $T_4$ value by this method was found to have a highly significant positive correlation with a free $T_4$ index (r; 0.957, p<0.005) and total $T_4$ (r:0.89, p<0.005) also, it was similar to other free $T_4$ measuring methods previously reported. So, it was showen to have the clinical utility as the screening test for thyroid function because it was a rapid; convinient, and reliable method for quantifying the free $T_4,;T_3$ concentration in various thyroidal functional states.