Comparison of in vitro Serum Protein Binding of Thyroxin and Triiodothyronine.

1. In vitro experiments on the serum protein binding of thyroxin and triiodothyronine were performed using paper electrophoresis to separate the protein fractions and 1131 labelled triiodothyronine and thyroxin to trace the localization of these amino acids. 2. Although both thyroxin and triiodothyronine were shown to be bound by alpha globulin, thyroxin was more firmly bound and not easily displaced by triiodothyronine whereas triiodothyronine was less firmly bound and readily displaced by thyroxin. 3. Displaced triiodothyronine was found to trail the thyroxin-binding globulin, whereas displaced thyroxin was seen to associate with the albumin fraction. 4. These observations are possibly related to the rapid disappearance of triiodothyronine after intravenous injection and to the relatively low levels of triiodothyronine in blood.