Determination of thyroxine-binding globulin. A simplified procedure utilizing dextran-coated charcoal.

A method for determining thyroxine-binding globulin (TBG) concentration as the total thyroxine-binding capacity, has been developed. Prior electrophoretic separation of the three serum thyroxine-binding proteins is not required. Serum samples are diluted with a barbital-salicylate buffer pH 8.6, which inhibits thyroxine-binding by prealbumin. After incubation with 100 g/10O ml thyroxine containing 1311_ thyroxine, dextran-coated charcoal is added to the sample, which binds all the thyroxine not bound to TBG. The radioactivity in the supernatant solution is directly related to the concentration of TBG present. The Pearson’scorrelation coefficient between the TBG results for this new assayand the polyacrylamide electrophoretic assay is 0.964. The mean TBC concentration and standard deviation for 80 normals was 19.2 ± 2.6 g/10O ml. Pregnant women and women taking estrogens had a mean and standard deviation of 35.8 ± 4.8 g/10O ml. Males from TBG-deficient families had TBG valuesof 5 l.g/100 ml or less, and females from these families had values ranging from 8 to 12 p.g/100 ml. The new assay is considerably simpler in equipment requirements and technic than the assays currently being used, and shouldbe more practical for routine clinical laboratory use.