A new radioimmunological method for determining free thyroxine

Abstract

A new method for determining free thyroxine in human serum was evaluated. It is a radioimmunological direct assay method using an anti-thyroxine antiserum which is very specific and bound to a solid-phase. To determine the accuracy of the method, we measured ‘with-in-assay’ reproducibility to have a CV% maximum of 2.9, and ‘between-assay’ reproducibility (for three sera with three different FT4 levels) to have a mean CV% of 8.2; the reproducibility of the standard curves in 12 different assays showed a maximum CV% of 9.2. FT4 levels were also evaluated in 167 subjects: in 55 normal subjects a mean FT4 value equal to 13.99 ng/l (range 8.8–19.1) was obtained; in 25 euthyroid subjects with high TBG and in 5 with low TBG normal values were obtained in 97% of the cases, whilst FTI values were within normal limits in only 77%; in 25 euthyroid patients with goitre, FT4 values were always within normal limits, in contrast with TT4 and FTI values (92%); in 25 patients with hyperactive ‘hot’ thyroid nodules, values which were on average higher (15.2 ng/l) than the range were obtained in 24% of the cases, whilst TT4 and FTI were higher than the range in 12% of the cases; in hyperthyroid (no.=24) and hypothyroid patients (no.=16), FT4 values were not within the normal range. The assay method was compared with another commercial method (radioimmunological, after separation of bound and free fractions by chromatography on Sephadex). There was a highly satisfactory correlation between the two methods (y=4.12+0.96x, r=0.982) in 123 samples, although a tendency was observed for the new method to show higher values than the older. We conclude by stating that the method studies is convenient and has acceptable accuracy and clinico-diagnostic reliability.