Eric Paul Krenning

Thyroid Hormone-Binding Plasma Proteins

The thyroid hormones thyroxine (T4) and 3,3′,5-triiodothyronine (T3) are bound to serum proteins for more than 99%. These proteins are thyroxine-binding protein (TBG), thyroxine-binding prealbumin (TBPA), and albumin. TBG is the most important T4 transport protein as it binds about 75 to 80% of T4, while both TBPA and albumin bind approximately 10% (1). There is no unanimity in the literature with regard to the exact serum distribution of T3 over the serum proteins, however probably also here TBG binds most of it (2). In this short review some physico-chemical characteristics of these proteins will be discussed, as well as their interaction with T4 and T3 and in the case of TBPA also its interaction with retinol-binding protein. In addition, the genetic aspects will be reviewed.