[Measurement of thyroxine binding globulin by competitive ligand binding assay (CLBA) (author's transl)].

The competitive ligand binding assay (CLBA) first described by Chopra et al. ((1972) J. Clin. Endocrinol. Metab. 35, 565-573) is a convenient routine method for the accurate measurement of thyroxine binding globulin in large numbers of serum samples. The assay is based on the partition of a constant quantity of radiolabelled T3 between a fixed quantity of rabbit T3 antibodies and the thyroxine binding globulin of the serum, after prior removal of T3 and T4 from the serum with an anion exchange resin (Amberlite IRA 400). In euthyroid subjects serum thyroxine binding globulin was 25.5 +/- 5.0 mg/1, in hyperthyroid patients thyroxine binding globulin was significantly decreased to 13.0 +/- 4.0 mg/1 and was significantly increased in hypothyroid patients to 36.8 +/- 6.2 mg/1 as well as in pregnant women to 41.3 +/- 6.2 mg/1. No difference was found between normal subjects and young women taking contraceptive pills with low oestrogen content. There were significant negative correlations between the thyroxine binding globulin in serum on the one hand side and the free T4-and free T3-fraction on the other. The low thyroxine binding globulin estimates in hyperthyroid patients increased gradually to normal during treatment with thyroid blocking drugs, the elevated thyroxine binding globulin in hypothyroid patients decreased to normal during treatment with thyroid hormones. The competitive ligand binding assay used here seems to be convenient as a routine method for the precise and reproducible measurement of thyroxine binding globulin in serum.