Recent Achievements in Studies on Thyroid Hormone-Binding Proteins*

THYROID hormones circulate in the bloodstream mostly bound to a set of plasma proteins which widely differ in their concentration and affinity for the hormones (1). The three major transport proteins are T4-binding globulin (TBG), T4-binding prealbumin (TBPA), and albumin. TBG carries about 70% of T4 in the circulation and, therefore, represents physiologically the most important T4-binding protein (1). 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): also the 27 K protein identified in normal human plasma and synthesized by human liver cell in culture (5–7) would appear to be apolipoprotein A-l (8). In the last few years, several papers have provided more precise information concerning TBG biosynthesis, the effects of its interaction with the carbohydrate moieties and with the ligand (T4), factors controlling TBG synthesis, TBG gene structure, and a number of genetic variants of the molecule. The present paper is mainly devoted to a review of these recent achievements, with the exception of inherited TBG abnormalities, which are extensively covered in a previous review by Refetoff (9). Additional sections deal with recent findings providing new information on the other major thyroid hormonebinding proteins, TTR and albumin.