[Assay of the free fraction of thyroxine. Its role in the study of thyroid function]

In a prospective study of 467 subjects with normal, increased or decreased thyroid function the results of free thyroxine (FT4) assays by 2 radioimmunological methods were compared with the free thyroxine index (FT1) and the T4/TBG ratio. It appeared from this study that FT4 assays were at least as good as the FT1 to confirm the diagnosis of euthyroidism and were distinctly superior to measurements of FT1, T4 adn T3 in patients with abnormal thyroid function. In view of the close correlation observed between clinical findings and the results of FT4 assays, a new strategy may be proposed to investigate thyroid function. However, further studies are required to evaluate the reliability of FT4 assays in some severe pathological conditions or in patients taking drugs interfering with thyroid hormone metabolism.