[Direct determination of free thyroxine in two simultaneous radioimmunoassays with different test principles: validity of test research derived from an area of marked iodine deficiency (author's transl)].

Serum FT4 was measured simultaneously by the commercial FT4 kits of Amersham-Buchler and Corning Medical in 4 groups with defined thyroid function as follows: 57 euthyroid subjects with and without goitre, 27 patients on T4 supplementation, 12 subjects on oral contraceptives and 26 subclinically hyperthyroid patients. All patients underwent physical examination, thyroid scintigraphy and additional in-vitro tests (total thyroxine T4, total triiodothyronine T3 and TRH/TSH assay). Both assays generally correlated well with the defined thyroid functional status except for the patients on oral contraceptives. Contrary to the FT4-test (Amersham-Buchler) in this group the FT4-test (Corning Medical) produced a significantly increased mean for FT4 as compared to the euthyroid groups thus indicating a probably insufficient compensation for high but not necessarily abnormal levels of TBG concentration. Using the FT4-assay (Amersham-Buchler) the normal range of FT4 values of euthyroid subjects from the area of Bamberg with marked iodine deficiency agrees well with recently published results of a multicentre trial thus confirming the assumption of serum FT4 not being strongly dependent on iodine intake. The results of the FT4-test (Corning) showed a wider scatter of normal ranges of FT4 values for all groups and concomitantly reduced discrimination compared to the corresponding simultaneously obtained FT4-test (Amersham-Buchler) results. Though cost and time saving, the commercial radioimmunoassays will require further critical evaluation of normal range validity on larger populations of defined abnormalities of thyroid function to elucidate inherent deficiencies of the new FT4 methods and of their diagnostic efficiency.