Clinical evaluation of a thyroxine binding globulin assay in calculating a free thyroxine index in normal, thyroid disease, and sick euthyroid patients.

As assay for thyroxine binding globulin (TBG) that measures the thyroxine binding capacity of TBG and other proteins was used to calculate a free thyroxine index (FTI) in 108 consecutive thyroid patients, 2,559 normal patients, and 152 sick euthyroid patients. The TBG assay compared favorably with the triiodothyronine (T3) uptake test in producing a FTI for the clinical evaluation of thyroid patients. In addition, it did not suffer the disadvantages inherent in assays specific for only TBG. In the sick euthyroid population, the TBG assay produced an FTI that was more consistent with the clinical evaluation than did the T3 uptake test.