Thyroxine-binding globulin (TBG) is one of three proteins responsible for carrying the thyroid hormones thyroxine (T4) and 3,5,3’-triiodothyronine (T3) in the bloodstream, along with transthyretin and albumin. TBG has the highest affinity for T4 and T3, but is present in the lowest concentration of the three proteins. Despite its low concentration, TBG carries the majority of T4 in serum. Due to the very low serum concentration of T4 & T3, TBG is rarely more than 25% saturated with ligand. Unlike transthyretin and albumin, TBG has a single binding site for T4/T3
Genomically, TBG is a serpin, although it has no inhibitory function like many other members of this class of proteins. TBG is synthesized primarily in the liver as a 54 kDa protein.