Clinical Chemistry, Vol 15, 1132-1140, Copyright © 1969 by the American Association for Clinical Chemistry

Determination of Thyroxine-Binding Globulin

A Simplified Procedure Utilizing Dextran-Coated Charcoal

¹ Marshfield Clinic Foundation for Medical Research and Education and the Marshfield Clinic, 510 North St. Joseph Avenue, Marshfield, Wis 54449.

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 µg/100 ml thyroxine containing ¹³¹l-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’s correlation coefficient between the TBG results for this new assay and the polyacrylamide electrophoretic assay is 0.964. The mean TBG concentration and standard deviation for 80 normals was 19.2 ± 2.6 µg/100 ml. Pregnant women and women taking estrogens had a mean and standard deviation of 35.8 ± 4.8 µg/100 ml. Males from TBG-deficient families had TBG values of 5 µg/100 ml or less, and females from these families had values ranging from 8 to 12 µg/100 ml. The new assay is considerably simpler in equipment requirements and technic than the assays currently being used, and should be more practical for routine clinical laboratory use.