Use of direct thyroxine-binding globulin measurement in the evaluation of thyroid function.

Abstract

Serum thyroxine-binding globulin (TBG) was measured by radioimmunoassay in 295 euthyroid subjects (220 normals and 75 estrogen-treated subjects), in 21 hyperthyroid and 19 hypothyroid patients. In normals, TBG concentration was 1.61 ± 0.29 mg/100 ml (mean ± SD) and a 71% increase was found in estrogen-treated subjects. Hyperthyroid patients had a normal mean TBG and hypothyroid patients showed slightly elevated levels. The usefulness of direct TBG measurement for the evaluation of thyroid function was assessed by comparing the results of T3 resin uptake tests (T3RU) performed in the same subjects. Three lines of evidence demonstrated the advantages of TBG measurement, i) In euthyroid subjects with elevated TBG levels T3RU results underestimated the actual TBG changes, ii) T4 levels correlated more closely with TBG than with T3RU results, iii) T3RU results, depending both on the concentration of hormone and binding protein were found less accurate for the adjustment of total T4 concentrations. Serum TBG and T4 values were used to calculate the free T4 (FT4) concentration. The FT4 was 1.2 ± 0.3 ng/100 ml in normals and the FT4 distribution in estrogen-treated subjects was entirely within the limits of normal subjects. In hypothyroidism FT4 was 0.3 ± 0.2 ng/100 ml and in hyperthyroidism 3.1 ± 1.1 ng/100 ml. Calculated FT4 was compared to direct FT4 measurements in 97 subjects and gave almost identical results. In conclusion, serum TBG can be reliably measured by radioimmunoassay. Adjustment of total T4 concentrations should be performed with TBG-based indices rather than T3RU. Free T4 concentrations can be obtained accurately from total T4 and TBG levels in euthyroidism as well as in pathological states of thyroid function.