[Clinical significance of free serum thyroxine--comparative studies with 5 methods]

In order to check whether the serum free T4 as obtained with commercially available kits can substitute for total T4 measurements in the routine diagnosis of thyroid disorders, serum FT4 was simultaneously assayed using 5 different methods (Amerlex; GammaCoat; Immo-Phase; LiquiSol; T4/TBG) in 152 subjects. Overt hyper- and hypothyroidism were readily distinguished from euthyroidism with each method. Discrepant FT4 results, however, were obtained with these assays in clinically euthyroid subjects exhibiting various extrathyroidal disorders (TBG-deficiency or -excess, late pregnancy, serious illness, T4 administration). Since it is as yet impossible to decide which of the commercial kits will yield methodologically and physiologically reliable results, a substitution of total T4 measurements by FT4 cannot yet be recommended in the laboratory diagnosis of thyroid disease.