Clinical evaluation of free thyroxine in serum by direct radioimmunoassay

This study was undertaken to assess a new and simple procedure (Amerlex Free Thyroxine assay) for the evaluation of free thyroxine (FT4) in serum. The principle of this single test relies on the competition between serum FT4 and a 125I-labelled analog of T4 (claimed to have negligible affinity for the T4-binding proteins) for a high-affinity antiserum to T4 bound to uniform microparticles. A total of 243 subjects, distributed among nine groups, including controls, various thyroidal conditions, and severe non-thyroidal illnesses were studied. The data correlated well with clinical observations and the usual routine tests of thyroid function. In particular, a similar pattern of values was observed in euthyroid groups: controls, non-toxic goitres, oestrogens, T4 therapy, and severe non-thyroidal illnesses. No overlap between controls and hypo- or hyperthyroid patients was observed; a good accuracy in the patients' diagnosis was obtained since only two patients with T3-thyrotoxicosis were wrongly classified by the FT4 test. As a whole, comparison with the FT4 index (T4/TBG ratio) was fairly good. However, discrepancies between FT4 and the index occurred for high FT4 values and for low TBG (thyroxine-binding globulin) values. The reason of these observations is discussed. (author)