Clinical Chemistry, Vol 10, 136-145, Copyright © 1964 by the American Association for Clinical Chemistry

A Study of Variables Affecting the Ion-Exchange Resin Uptake of I¹³¹-Labeled Triiodothyronine from Human Serum

¹ Bio-Science Laboratories, Los Angeles, Calif.

When serum is incubated with I¹³¹-labeled triiodothyronine and then equilibrated with the anion-exchange resin IRA-400 in the formate cycle, a fraction of the total radioactivity is bound to the resin. Resin uptake values are reproducible and have been correlated with alterations in the thyrometabolic status of patients. A study of a number of fundamental variables in this test yielded the following observations. Resin uptake values were found to be linearly related to serum thyroxine concentration over a limited range; no comparable relationship could be shown for triiodothyronine. Of the specific thyroxine-binding proteins in serum, only the interalpha globulin fraction could be shown to be involved. It was also observed that the radioactivity ultimately bound to resin had been initially associated with serum protein and was continuously being translocated to resin as equilibration time was extended. This continuous translocation was multiphasic with respect to time and probably represented the result of several equilibrium compartments, each involving one or more different protein species. Consequently, these findings indicated that those proteins which bound triiodothyronine nonspecifically had a distinct role in determining resin uptake values.