Clinical evaluation of two direct procedures for free thyroxin, and of free thyroxin index determined nonisotopically and by measuring thyroxin-binding globulin

We have investigated the clinical utility of two direct radioimmunoassays for free thyroxin, an enzyme-inhibition immunoassay, and a direct measurement of thyroxin-binding globulin (TBG) by radioassay. All assay method correctly identified ≥90% of euthyroid, hyperthyroid, and hypothyroid patients who had normal TBG concentrations. In patients with altered TBG concentrations, none of the assays correctly classified all categories of patients. However, the direct assays of free thyroxin concentrations were able to classify correctly more patients with altered TBG concentrations than did the free thyroxin index methods. The free thyroxin index methods evaluated may be acceptable for routine use, if the concentration of thyroxin and the measurement of TBG capacity are reported along with the index value. Patients with altered TBG concentrations included a group of euthyroid pregnant patients. Significant decreases in free thyroxin in the third trimester were detected by all the assays studied. For patients in the first and second trimester, the mean free thyroxin concentration measured varied with the assay method.