Thyroxine by displacement analysis (T4D) along with the resin-T3 (RT3) and resin-T4 (RT4) ratios were determined analytically on serum samples from 160 ‘normal’, 50 hypothyroid and 70 hyperthyroid persons. From these data, the mathematical products of the T4D and either the RT3 or RT4 ratios were calculated and then designated the free thyroxine indices (FTI-T3 and FTI-T4). These were compared to the mean value ± 3 SD range derived from 137 normal blood bank samples. The T4D used alone was the single most accurate diagnostic parameter. The RT3 and RT4 were singularly poor tests for discriminating thyroidal dysfunction from normal. Despite theoretic considerations, the FTI-T4 was superior in diagnostic discrimination compared to the FTI-T3 with 93.8% ‘normal’ but only 16.0% hypothyroid and 0.0% hyperthyroid values in the normal range. The FTI-T4 showed a linear correlation with the free thyroxine concentration obtained by equilibrium dialysis. The FTΓ-T4 is particularly advantageous for clinical use since it is derived from the T4D which obviates interferences from iodine contamination and the RT4 which can be performed in concert with the T4D by a unified procedure.