Thyroxin-binding globulin

The thyroid hormones are lipophilic and relatively insoluble in the plasma. Therefore, they are transported throughout the circulation bound to plasma proteins such as thyroxine-binding globulin (75%) and albumins (25%). Approximately 99.96% of circulating thyroxine is protein bound. Bound hormone is not available to cause any physiological effects however, it is in equilibrium with the remaining 0.04% that is unbound. This free form of the hormone is able to bind to receptors on target tissues and cause its effects. Thyroid hormone has many metabolic effects in the body ... 

Thyroxine binding globulin 0.01-0.02 58 Binds/transports thyroxine... 

Estrogen may increase hepatic production of thyroxine-binding globulin (TBG) and decrease TBG clearance, thus increasing serum total thyroxine (tT4) and, to a lesser extent, total triiodothyronine (tT3). As a result, increased tT4 and tT3 are seen in states of excessive estrogen and/or progestin, such as... 

Duntas LH, Mantzou E, Koutras DA (2001) Lack of substantial effects of raloxifene on thyroxine-binding globulin in postmenopausal women dependency on thyroid status. Thyroid 11 779-782... 

The basis for this technique lies in the competition between the test antigen and a labelled antigen for the available binding sites on a fixed amount of antibody. While the binding sites are traditionally associated with an antibody, any source of specific reversible binding sites may be used to create an assay in this format. Examples of such are specific transport proteins such as thyroxine-binding globulin and certain cellular receptors such as opiate or benzodiazepine receptors. Under these circumstances the equilibrium mixture may be represented thus ... 

PAGE Poiyacryiamide gei eiectropho- TBG Thyroxine-binding globulin... 

P5. Pensky, J., and Marshall, J. S., Studies on thyroxine-binding globulin (TBG)... 

Thyroid function tests - Decreased levels of thyroxine-binding globulin, resulting in decreased total T4 serum levels and increased resin uptake of T3 and T4. Free thyroid hormone levels remain unchanged, and there is no... 

Thyroxine-binding globulin is the least abundant of the three major transport proteins. Nevertheless, it carries about 70% of the circulating T4 and Tj by virtue of its high affinity for the two hormones. Transthyretin, formerly known as thyroxine-binding prealbumin, binds only about 10 to 15% of the hormones. Albumin, a protein that has a binding affinity for a multitude of small molecules, has an even lower affinity for T4 and T3 than... 

The iodothyronines are very insoluble molecules and are kept in solution by transport proteins. The most important of these is thyroxine binding globulin (TBG), which carries about 65% T and 70% Tj. It is a small (MW 60,000-65,000) glycoprotein consisting of four subunits. It has a single, high-affmity binding site for T, with an estimated of 1.2 x 10 °M. 

T4 and T3 in plasma are reversibly bound to protein, primarily thyroxine-binding globulin (TBG). Only about 0.04% of total T4 and 0.4% of T3 exist in the free form. Many physiologic and pathologic states and drugs affect T4, T3, and thyroid transport. However, the actual levels of free hormone generally remain normal, reflecting feedback control. 

Lans MC, Spiertz C, Brouwer A, et al. 1994. Different competition of thyroxine binding to transthyretin and thyroxine-binding globulin by hydroxy-PCBs, PCDDs and PCDFs. Eur J Pharmacol 270 129-136. 

Thyroid hormones and most steroid hormones are associated with carrier proteins in the serum. The carrier proteins are called, appropriately, thyroxine-binding globulin, transcortin (for cortisol), and sex-steroid-binding protein. These proteins have a high affinity (Kd 10—9— 10 8 m) for their respective hormones. They buffer the concentration of free hormone and retard hormone degradation and excretion. The carrier proteins are distinguishable from the intracellular receptors for these hormones. 

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