Enzyme immunoassay of thyroxin-binding globulin.

We describe a new double-antibody enzyme radioimmunoassay for thyroxin-binding globulin (TBG) in serum. A TBG-beta-D-galactosidase complex was prepared with m-maleimidobenzoyl N-hydroxysuccinimide ester. The mean analytical recovery of TBG added to serum was 99%, sample volume and TBG value were linearly related, and values for TBG determined by this method and those determined by radioimmunoassay correlated well (r = 0.98). Coefficients of variation averaged 7.6% within assay, 6.6% between assay. The measurable range of TBG in the serum was 3.3 to 52 mg/L, but higher TBG concentrations could be measured by diluting the test serum with TBG-free serum. Mean (and SD) serum TBG concentrations as determined by this method were as follows: for 20 normal subjects, 21.2 (3.7) mg/L; for 10 pregnant women, 50.1 (9.5 mg/L; and for 10 patients with TBG deficiency, not detectable.