Evaluation of a new enzyme immunoassay system for free thyroxine (Enzymun-Test FT4).

Free thyroxine (FT4) represents the metabolically active fraction of the circulating thyroid hormone thyroxine (T4). In this paper the results of the evaluation of a newly developed FT4 assay are reported. This assay system is based on an enzyme-labelled one-step immunoassay. The within-run imprecision, checked using serum pools and several commercial reference materials, showed a coefficient of variation (CV) of between 0.8 and 9.8%, depending on the reference material used. The between-run imprecision showed a CV of between 1.0 and 13.2%. Accuracy experiments yielded values between 80 and 116%. When the new FT4 was compared with the calculation of an index for free thyroxine (FT4I; derived from either the ratio of T4/thyroxine binding coefficient of from T4/thyroxine binding globulin) in a number of samples in the hypo-, eu- and hyperthyroid range, a good correlation was obtained. The same was true when the new FT4 assay was compared with a widely used two-step radioimmunoassay (y = -0.146 + 0.943 x). In euthyroid subjects the measured FT4 concentration was 10.3-25.8 pmol/l. No effect was evident when the influence of EDTA and citrate was investigated, whereas addition of heparin led to an increase in FT4 concentration of about 12 to 15%. Investigation of the possible influence of a large number of drugs showed that probenezid, carbamazepine and furosemide led to an increase in the measured FT4 concentration. Dialysis increased the FT4 concentration, as measured in patients before and after haemodialysis. No effect of alteration in protein concentration and/or protein distribution of FT4 concentration could be detected. In pregnancy, FT4 values were within the normal range.(ABSTRACT TRUNCATED AT 250 WORDS)