Immunoassay of the free fraction of T4. Comparative study of three nonisotopic and two two-step isotopic immunoassays

Radioimmunoassays (RIA) of the free fraction of thyroxin using labeled analogs give artefacts in certain disorders now well known. Three nonisotopic (Amerlite, Magic Lite, Stratus) and two two-step isotopic immunoassays (Berhing and Clinical Assays), are studied by comparison with two FT4 Radioimmunoassays: Amerlex and Sclavo. With the all methods, results were similar to those obtained by the Sclavo assay: intra and inter assays coefficients of variation less than 10% in the working range and no significant interaction between tracer and thyroxin binding protein. In a population of 325 euthyroid subjects, 111 hyperthyroid and 61 hypothyroid patients without any treatment, slight overlaps were observed between the hyperthyroid and euthyroid populations and diagnostic sensitivities were for hyperthyroidism and hypothyroidism about 0.95 and 0.80. Results were comparable to those obtained with the Sclavo assay in specific populations in which FT4 concentrations are known not to be always related to their euthyroid status (69 patients with chronic renal failure, 20 patients in intensive-care, 17 patients treated with heparin, and 27 pregnant women). The methods studied gave globally good results and were better than the one-step analog based method