Thyroxine-binding globulin

Overview
Thyroxine-binding globulin (TBG) is one of three proteins (along with transthyretin and 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 serum. Due to the very low serum concentration of T4 & T3, 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 produced in the liver.

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.