Thyroxine-binding globulin (TBG) is a globulin protein encoded by the SERPINA7 gene in humans. TBG binds thyroid hormones in circulation. It is one of three transport proteins (along with transthyretin and serum albumin) responsible for carrying the thyroid hormones thyroxine (T<sub>4</sub>) and triiodothyronine (T<sub>3</sub>) in the bloodstream. Of these three proteins, TBG has the highest affinity for T<sub>4</sub> and T<sub>3</sub> but is present in the lowest concentration relative to transthyretin and albumin, which also bind T3 and T4 in circulation. Despite its low concentration, TBG carries the majority of T<sub>4</sub> in the blood plasma. Due to the very low concentration of T<sub>4</sub> and T<sub>3</sub> in the blood, TBG is rarely more than 25% saturated with its ligand. Unlike transthyretin and albumin, TBG has a single binding site for T<sub>4</sub>/T<sub>3</sub>.