Inherited Abnormality of Thyroxine-Binding Globulin with No Demonstrable Thyroxine-Binding Activity and High Serum Levels of Denatured Thyroxine-Binding Globulin

THYROID hormones (thyroxine and triiodothyronine) are transported in blood bound to serum proteins.1 2 3 The principal protein, which carries approximately 75 percent of the total thyroxine and triiodothyronine in serum, is thyroxine-binding globulin (TBG), an acidic glycoprotein of hepatic origin:4 , 5 Although wide changes in the concentration of TBG do not alter metabolic status, they do affect the concentration of thyroxine and triiodothyronine in serum.Five distinct types of inherited TBG variants have been described in humans. All follow an X-linked mode of inheritance.6 7 8 9 The three TBG variants that are believed to be quantitative are widely distributed among ethnic groups and all . . .