Clinical value of total T4 and T3 determinations in patients with suspect hyperthyroidism before and after correction for binding proteins.

Abstract

ABSTRACT. In a previous Investigation we found that the determination of serum T3 by a radioimmunoassay procedure was superior to both the determination of serum T4 and the TRH test for laboratory discrimination between eu‐ and hyperthyroidism. The investigation comprised 50 patients in whom hyperthyroidism could not be excluded by the first clinical examination alone. The complete investigation showed that 26 of the patients had hyperthyroidism and 24 had normal thyroid function. The aim of the present study was to elucidate whether the discriminatory power of T3 and T4 estimations in these patients could be increased by correction for variations in the hormone‐binding proteins. The influence of the proteins was estimated by the T3 uptake test and determination of the TBG level by radioimmunoassay technique. The free T4 and T3 indices were calculated according to standard procedure. The results demonstrate that all subjects had TBG values within the reference levels. In the discrimination between normal and hyperthyroid states, the kee T4 index gave more reliable results than the uncorrected T4 or the T4: TBG ratio. However, the discriminatory power of T3 was superior to both T4, T4: TBG ratio and free T4 index. No improvement was obtained by calculation of the T3: TBG ratio. A slight improvement was obtained by calculation of the free T3 index. It is concluded that estimation of the total serum T3 level is superior to the total serum T4 level in the laboratory discrimination between euand hyperthyroidism even subsequent to correction for the binding proteins.