Radioimmunoassay of free thyroxine with prebound anti-T4 microcapsules.

Free thyroxine (FT4) may be one of the active throid hormones in contact with target end organs. It is unaffected by alterations in serum protein levels. In most cases, measurement of FT4 reflects an individual's true thyroid function or dysfunction. Previous FT4 assay techniques have been difficult, tedious, indirect, and inaccurate. A rapid, simple, and accurate radioimmunoassay for FT4 has been developed using microencapsulated rabbit anti-T4 antiserum to which I-125 T4 tracer of high specific activity has been complexed. Addition of FT4 standards or unknown samples displaces a proportional amount of I-125 T4 from antibody. Protein-bound T4 is excluded from the reaction by short incubation time and spatial configurations. Specimens representing known thyroid dysfunction were tested using the above procedure. The normal range of FT4 was 0.8-2.4 ng/dl. The mean FT4 for the hyperthyroid group was 6.92 +/- 1.38 (range 4.4-9.6) ng/dl. The mean FT4 for the hypothyroid group was 0.43 +/- 0.37 (range 0.1-1.3) ng/dl, and in pregnancy the mean FT4 was 1.64 +/- 0.44 (range of 1.0-2.2) ng/dl (1).