Thyroid hormone binding plasma proteins

Binding of thyroid hormones to plasma proteins protects the hormones from metabolism and excretion and prolongs their half-lives in circulation. The free (unbound) hormone is a small percentage (about 0.03% of T and 0.3% of Tj) of the total hormone in plasma. The differential binding affinities for plasma proteins also are reflected in the 10-100-fold differences in circulating hormone concentrations and half-lives of T and T. ... 

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,... 

Steroid and thyroid hormones bind to specific honnone binding glycoproteins in plasma. It is the unbound or free fraction of the hormone in plasma which is biologically active so measurement of free hormone status or binding protein levels may be important in the diagnosis of patients with thyroid or gonadal disorders. 

When thyroid hormone results are interpreted, reference should be made to the plasma protein concentrations because of their thyroid hormone binding properties and the effects on potential protein binding sites (Panciera and Johnston 2002). 

Thyroid hormones are taken into cells by facilitated diffusion or by active transport secondary to a sodium gradient (11). Once in the cell, thyroid hormones bind to cytosolic binding proteins and are not readily available for exchange with plasma hormones. Both T3 and T4 are not evenly distributed in body cells A great part of T4 is... 

The resin uptake of labeled T3 (T3U), an indirect parameter for assessing the levels of thyroid hormone in plasma, is based on an earlier technique, namely, the red blood cell uptake of added I-labeled Ts from plasma, Hamolsky et al. (H9) developing the latter technique in 1957. The principle of the test is that T3 is partitioned between the red blood eells and the plasma, depending on the occupying of specific binding sites on the plasma proteins with endogenous thyroid hormone. In other words, the uptake of T3 by the cells is inversely proportional to free TBP capacity. It was soon realized that the red blood cell is a variable in the system, which affected the final result (R3) and the cell was replaced with a fixed amount of ion-exchange resin (M13). 

CIS. Clark, F., The estimation of thyroid hormone binding by plasma proteins and of unbound levels of thyroxine in plasma. J, Clin. Pathol., SuppL 20, 344-352 (1967). 

Only small amounts of free T are present in plasma. Most T is bound to the specific carrier, ie, thyroxine-binding protein. T, which is very loosely bound to protein, passes rapidly from blood to cells, and accounts for 30—40% of total thyroid hormone activity (121). Most of the T may be produced by conversion of T at the site of action of the hormone by the selenoenzyme deiodinase (114). That is, T may be a prehormone requiring conversion to T to exert its metaboHc effect (123). 

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. 

Steroid and thyroid hormones are minimally soluble in the blood. Binding to plasma proteins renders them water soluble and facilitates their transport. Protein binding also prolongs the circulating half-life of these hormones. Because they are lipid soluble, they cross cell membranes easily. As the blood flows through the kidney, these hormones would enter cells or be... 

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 ... 

The same 17 PBDE congeners and three hydroxylated PBDEs were also tested for possible interaction with T4 binding to human TTR, a plasma transport protein of thyroid hormones, in an in vitro competitive binding assay (Meerts et al. 1998, 2000). None of the pure congeners competed with T4 for binding to... 

Lipid-soluble hormones act usually by gene activation/deactivation. Examples of these hormones include steroids, thyroid hormone, and vitamin A (retinoic acid). The hormones are transported through the circulation in association with a hormone-binding protein and are soluble in the plasma membrane of the cell. Their receptors are intracellular, and they act on gene transcription (the synthesis of messenger RNA) rather than at the protein level. Thus, they act more slowly than do the soluble hormones, on the scale of days rather than minutes. 

Several proteins, with binding activities for thyroid hormones, have been detected in a variety of cell types and with different subcellular localizations, i.e., in the plasma membrane, the sarcoplasmic reticulum, the cytosol, the mitochondria and... 

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