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