Radioimmunoassay of thyroxine-binding globulin: evaluation of a kit and diagnostic application.

Serum thyroxine-binding globulin (TBG) was measured by using a radioimmunoassay method. The within-batch coefficient of variation (CV) of the assay was 5% or less at different TBG concentrations with a between-batch variation of less than 10%. The mean TBG concentration in hypothyroidism (28.5 +/- 6.7 mg/l) was significantly higher, and that in hyperthyroidism (22.3 +/- 3.5 mg/l) was significantly lower, than the euthyroid mean (25.4 +/- 6.9 mg/l). Acutely ill patients suffering from various non-thyroidal illnesses were found to have significantly low mean TBG (13.8 +/- 4.3 mg/l). A group of women in late pregnancy had a higher mean value, as expected. Serum TBG level, however, was ineffective as a thyroid function test because of overlapping concentrations between groups. The derived index T4:TBG ratio was found to be superior to free thyroxine index (FTI) in many cases of acutely ill patients, pregnancy, and hereditary TBG abnormality. The 95% confidence limit for the ratio in euthyroids was 2.3 to 7.1. Serum TBG level with T4:TBG ratio is recommended as a replacement for the T3-uptake test and FTI.