Thyroid-binding prealbumin

Thyroid hormone is liberated into the bloodstream by the process of proteolysis within thyroid cells. T4 and T3 are transported in the bloodstream by three proteins thyroid-binding globulin, thyroid-binding prealbumin, and albumin. Only the unbound (free) thyroid hormone is able to diffuse into the cell, elicit a biologic effect, and regulate thyroid-stimulating hormone (TSH) secretion from the pituitary. 

The concentration of free rather than bound thyroxine is considered to be the most accurate assessment of thyroid activity as this is the fraction which can penetrate cell membranes and exert a metabolic effect. Free thyroxine exists in equilibrium with thyroxine bound to globulin, albumin, and prealbumin. Any changes in the concentration of thyroid binding proteins leading to an increase in free hormone. Thyroid binding prealbumin is reduced after all kinds of stress and the reduction is significant within 24 hours. 

TBG thyroid-binding globuhn TBPA thyroid-binding prealbumin TCO4 pertechnetate TG thyroglobulin... 

Thyroid-binding prealbumin (TBPA) has been easier to isolate and purify because it can be separated from other proteins by electrophoretic methods and because it is present in greater quantities (Table 2). Purified TBPA is immunologically pure as well as homogeneous on ultracentrifugation and electrophoresis (013). The molecular weight is about... 

Thyroxine (3, 5, 3,5-L-teraiodothyronine, T4) is a thyroid hormone, which is transformed in peripheral tissues by the enzyme 5 -monodeiodinase to triiodothyronine. T4 is 3-8 times less active than triiodothyronine. T4 circulates in plasma bound to plasma proteins (T4-binding globulin, T4-binding prealbumin and albumin). It is effective in its free non-protein-bound form, which accounts for less than 1%. Its half-life is about 190 h. 

The lodothyronines secreted by the thyroid gland into thyroid vein blood are of limited solubility. They equilibrate rapidly, however, through noncovalent association with three major binding proteins thyroid binding globulin (TBG), transthyretin (TTR formerly called T4 binding prealbumin), and albumin. Thyroid... 

Thyroid hormones have a lengthy life-span in the bloodstream — several days — probably because they are bound to proteins in the circulation. More than of T4 and T3 is bound to plasma proteins. These prolerns arc thyroid hormone-binding protein, transthyretin, and albumin. Most of the hormone is carded by thyroid hormone-binding protein. Transthyretin (from thyroid and retinol) occurs in a Irl complex with retinol-binding protein In the bloodstream. This complex serves to prevent the loss of retinol-binding protein, which is a small protein, in the urine. Transthyretin has also been called prealbumin, it binds T4 and not TJ,... 

Transthyretin (prealbumin) and retinol-binding protein (RBP) are transport proteins that migrate together as a 1 1 molecular complex. Transthyretin was originally named prealbumin because of its electrophoretic mobility it was renamed in 1981 to reflect its binding and transport of both thyroid hormones (thyroxine and triiodothyronine) and RBP. 

Ingbar, 8. H., Observations concerning binding of thyroid hormones by human serum prealbumin. J. Clin. Invest. 42, 143-160 (1963). 

In plasma, over 99.95% of T4 is transported bound to proteins. Thyroxine binding globulin (TBG) carries 70% of Tj, albumin approximately 25% and transthyretin (formerly called prealbumin) around 5%. Over 99.5% of T, is transported by the same proteins. It is the unbound, or free . Tj and Tj concentrations which arc important for the biological effects of the hormones, including the feedback to the pituitary and hypothalamus. Changes in binding protein concentration complicate the interpretation of thyroid homione results, e.g. in pregnancy. 

Larsson, M., T. Pettersson, and A. Carlstrom. 1985. Thyroid hormone binding in serum of 15 vertebrate species Isolation of thyroxine-binding globulin and prealbumin analogs. General Comparative Endocrinology 58 360-375. 

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