Thyroid-Binding Globulin Deficiency

Abstract

Thyroid hormones circulate in the blood stream mostly bound to a set of plasma proteins which widely differ in their concentration and affinity for the hormones. The three major transport proteins are thyroxine (T4) -binding globulin (TBG), T 4 -binding prealbumin (TBPA), and albumin. TBG encoded by the TBG gene on chromosome Xq22 is the major transport protein which carries about 75 per cent of T4 in the circulation 1 and, therefore, represents physiologically the most important T 4 -binding protein. TBPA has been named transthyretin (TTR) for the role it also plays in the transport of the retinol-binding protein. Less important proteins with thyroid hormone binding activity are lipoproteins 2-4 . Variations in concentration of carrier proteins of hormones may influence the effect of hormones and may cause confusion of interpretation of laboratory results. Inherited TBG deficiency is considered to represent X-chromosome linked inheritance with hemizygote affected males and heterozygote female carriers with intermediate values for T 4 and TBG 5 . Commercial test kits for free thyroxine (FT 4 ) may present considerably different results in conditions with TBG deficiency. When high level of measured FT 4 combined with normal thyrotropin (TSH) is found, TBG deficiency should be considered 6 .