The serine proteinase inhibitors (serpins) compose a superfamily of proteins with a diverse set of functions, including the control of blood coagulation, complement activation, programmed cell death and development. Serpins are secreted glycoproteins that contain a stretch of peptide that mimics a true substrate for a corresponding serine protease. Serpin family members include thyroxine-binding globulin precursor (TBG). TBG is a serum protein that transports thyroxine, carrying approximately 75% of circulating T4. Inherited de-fects in TBG are associated with three phenotypes based on the level of TBG in serum of affected hemizygous males: complete TBG deficiency (TBG-CD), partial TBG deficiency (TBG-PD) and TBG excess (TBG-E). TBG is expressed by the liver and secreted in plasma.
