Clinical aspects and diagnosis of thyroid hormone transport protein anomalies.

Abstract

The thyroid hormones T4 (thyroxine) and T3 (3, 5, 3′-triiodothyronine) circulate in the blood reversibly bound to proteins. The principal transport proteins are: T4-binding globulin (TBG), transthyretin (TTR), formerly called T4-binding prealbumin or TBPA, and albumin. These proteins differ widely in concentration and their affinity for the iodothyronines (Oppenheimer 1968; Robbins and Rall 1960; Robbins et al. 1986); collectively, they carry more than 99% of T3 and T4 in serum. Other thyroid hormone-binding proteins in serum are lipoproteins (Benvenga 1989; Benvenga et al. 1988,1989,1990), for example the 27 K protein, an apolipoprotein A-I, and the apolipoprotein B-100, which are both synthesized in the liver (Grimaldi et al. 1986; Bartalena et al. 1986a, b). Less important binding proteins are immunoglobulin M (Sviridov et al. 1991) and, in thyroid autoimmune disorders, thyroid hormone autoantibodies of the IgG class (Benvenga et al. 1987).