Clinical evaluation of new indicators of free thyroxine concentration and thyroxine-binding globulin capacity.

A rapid and simple method is described for the indirect measurement of free thyroxine (T4) concentration and maximal thyroxine-binding globulin (TBG) capacity applying a single kit of Thyopac-4. Initially, a value for total T4 is determined, after which the patient's serum is added to permit estimation of T4 corrected for abnormal TBG concentration. The ratio of increased radioactivity in the supernatant after the addition of patient's serum to that obtained using pooled normal serum is designated the TBG index, being directly related to the amount of TBG in the added serum. The index is significantly increased in hypothyroidism and pregnancy, and decreased in hyperthyroidism, correlating well with the TBG capacity measured by reverse-flow paper electrophoresis. A free T4 index (FTI) can be calculated from the values for T4 and TBG index, because the TBG index is reciprocally related to the serum uptake test (T3-resin). Values for FTI are significantly increased and decreased in hyper- and hypothyroidism, respectively, whereas they remain within the normal range in pregnancy and TBG deficiency. The FTI is shown to bear a nearly straight-line relationship to that calculated from separate estimations of T4 and the serum uptake test (T3-resin).