The present study was undertaken to examine serum concentration of free T3, T4 in various functional states of the thyroid by Amerlex RIA kit method, which uses unidentified T3, T4- labelled analogues, said to be unreactive with T3, T4-binding proteins in serum, together with an antibody that binds both analogue and T3, T4. The test method has been compared with free T4 index, which was product of total serum thyroxine and T3 resin uptake ratio. Free T4 value by this method was found to have a highly significant positive correlation with a free T4 index (r; 0.957, P〈0.005) and total T4 (r:0.89, p〈0,005) also, it was similar to other free T4 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 T4, T3 concentration in various thyroidal functional states.