Competitive Ligand-Binding Assay for Measurement of Thyronine-Binding Globulin (TBG)

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Abstract
A competitive ligand-binding assay (CLBA), based on partition of a constant amount of radiolabeled ligand (T3) between thyronine-binding globulin (TBG) and a fixed amount of a rabbit anti-T3, has been utilized to measure serum TBG concentration. The method is convenient, sensitive, precise and reproducible, and it accurately measures differences in TBG concentration. Serum albumin does not interfere in the assay and any effect of prealbumin (TBPA) is prevented by the use of barbital buffer. The slopes of the competitive binding curves with normal sera and those with elevated serum TBG were similar. Serum TBG concentration as measured by CLBA was 2.85 ± 0.08 mg/100 ml (mean ± sem) in 59 normal subjects; it was significantly higher in patients expected to have elevated serum TBG because of estrogen treatment, pregnancy or a genetic abnormality (4.60 ± 0.18, p < .001), and it was significantly lower in hyperthyroid patients (1.89 ± 0.17, p <.001). Serum TBG concentration in hypothyroid patients (2.66 ± 0.26 mg/100 ml) was not significantly different from normal subjects. The diminished serum TBG in hyperthyroidism increased gradually when hyperthyroninemia was controlled with propylthiouracil treatment. The estimates of serum TBG by CLBA correlated significantly (r 0.93, p <.001) with maximal T4-binding capacity of TBG over a wide range of TBG concentration. The correlation of CLBA with T3 resin uptake was also significant but the relationship was curvilinear. When concentration of TBG-like protein(s) was quantitated in animal sera, using human serum as the reference preparation, values comparable to normal human sera were obtained in sheep, cow and monkey. A TBG-like protein was present in lower concentration in dog sera and it was absent in guinea pig and rat sera.

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