Binding characteristics of thyroxin binding globulin in serum of normal, pregnant, and severely ill euthyroid patients.

Serum from normal, pregnant, and severely ill patients was stripped of endogenous thyroid hormones and diluted 1100-fold in barbital buffer. We then used it to study the binding characteristics of thyroxin binding globulin (TBG), noting significant differences in binding capacities among the groups. The mean (+/- SD) triiodothyronine/thyroxin ratio for binding capacity was 18 +/- 4 for normal subjects. The ratio was significantly increased in pregnant patients, 21 +/- 4 (p less than 0.05), and significantly lower in severely ill patients, 12 +/- 4 (p less than 0.05). When serum was diluted before assay, to give a uniform TBG concentration among groups, these apparent differences in binding characteristics were eliminated. It therefore is unlikely that different molecular species of TBG account for the variations in binding characteristics in these clinical states. Apparently, the distribution of thyroxin and triiodothyronine among the binding sites on TBG changes with variations in TBG concentration. This may explain the discrepancies observed in the concentrations of free thyroid hormones as estimated by various methodologies.