ABSTRACT
The bulk of thyroid hormone circulates in most vertebrates bound to serum proteins. These differ widely in their concentration and affinity for the hormones triiodothyronine and thyroxine (T4). The three major transport proteins are T4-binding globulin (TBG), T4-binding prealbumin, and albumin. TBG is functionally the most important T4-binding protein. Transthyretin (TTR) is a 55-kDa protein consisting of four identical subunits each containing 127 amino acid residues. The tetramer is arranged so as to form a double trumpet-shaped channel where two binding sites for T4 are located. Usually only one T4 molecule is bound to TTR since the interaction of the T4 greatly decreases the binding affinity for the second binding site. Inherited abnormalities of TBG are relatively common, affecting about 1 of 2,500 newborn males. These TBG defects are X chromosome-linked and expressed in hemizygotes as complete or partial TBG deficiency.
