[Two generations of FT4-radioimmunoassays--results in euthyroid patients and in patients on thyroid hormone therapy].

: In accordance with the TRH-test two generations of FT4-immunoassays were checked in order to find a substitute for the free thyroxine index. The euthyroid reference range of FT4 was between 0.69-1.91 ng/dl using GammaCoat (Clinical Assays) and 0.94-2.29 ng/dl using Amerlex (Amersham), respectively. In the euthyroid range, FT4 as measured by Amerlex was, on the average, 47% higher than the value obtained by using GammaCoat. During treatment by thyroid hormones and TSH-suppression following TRH application, Amerlex FT4 values were up to 59% higher than the GammaCoat FT4 values. During replacement therapy, the FT4 values obtained by Amerlex were within the euthyroid reference range so that a differentiation between substitution and suppression appears to be possible by means of determining FT4. An overdose was assessed by simultaneous determination of T3 and FT4. According to our results Amerlex FT4 meets the requirements of a modern radioimmunoassay and is suitable as a substitute for the free thyroxine index. For this purpose the earlier method of determining FT4 (GammaCoat) does not seem suitable.