Protein disulfide isomerase (PDI) is an acidic 55 kDa thyroid hormone-binding protein that appears to be identical to the glial-p55 protein. It has been suggested to be involved in disulfide-bond formation and isomerization, as well as the reduction of disulfide bonds in proteins. It may also play a role in the thyroid hormone-dependent regulation of actin polymerization and the degradation of the membrane-associated protein type II iodothyronine 5'-deiodinase (5'D-II).