Thyroid hormones binding protein

Bartalina L Thyroid hormone-binding proteins update 1994. En-docr Rev 1994 13 140. 

In a second section of this chapter we will review the available data on the different cellular thyroid hormone-binding proteins so far described and on the nuclear receptor. 

E. Protein Disulfide-Isomerasc as Thyroid Hormone-Binding Protein... 

Liver Storage and Release of Retinol Tissues can take up retinyl esters from chylomicrons, but most is left in the chylomicron remnants that are taken up into the liver by endocytosis. The retinyl esters are hydrolyzed at the hepatocyte cell membrane, and free retinol is transferred to the rough endoplasmic reticulum, where it binds to apo-RBP. Holo-RBP then migrates through the smooth endoplasmic reticulum to the Golgi and is secreted as a 1 1 complex with the thyroid hormone binding protein, transthyretin (Section 2.2.3). 

Chu et al. [81,82] observed changes in the thyroid when rats were administered PCDEs. Rosiak et al. [104] have lately reported that some PCDEs can cause hypothyroidism in rats. PCDEs 71 and 102, which have closest resemblance to thyroxine (T4), were shown to cause hypothyroidism in pregnant rats and PCDEs 71,102, and 153 in juvenile rats exposed in utero. PCDEs 71 and 102 have been fetotoxic in mice [92]. Rosiak et al. [104] suggested that, e.g., the inhibition of the release of T4 from thyroid follicular cells by PCDEs or binding of PCDEs to thyroid hormone binding protein, transthyretin, could decrease T4 levels. Triiodothyronine (T3) levels were not altered by PCDEs 71,102, and 153. 

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,... 

Many disorders are associated with thyroid hormone excess or deficiency in the absence of definable thyroid disease. These states of euthyroid hyperthyroxinemia or euthyroid hypothyroxinemia often result from alterations in the concentration of thyroid hormone—binding proteins, the actions... 

The key feature of this two-step method is that the labeled hormone is physically prevented from interacting with serum binding proteins. This ensures that antibody binding of tracer is governed solely by the free hormone concentration and not by changes in thyroid hormone-binding proteins. Test results obtained with two-step methods correlate well with those obtained by reference methods across a wide range of clinical disorders. 

Determination of Thyroxine-Binding Globulin AND Other Thyroid Hormone-Binding Proteins... 

TBG is the thyroid hormone-binding protein having the greatest affinity for T4. As such, it is very important in regulating the concentration of the FT4 hormone. Estrogen-induced TBG excess and congenital TBG deficiency are the most significant TBG abnormalities that affect the interpretation of thyroid function test results (see Box 52-1). 

Faix JC, Rosen HN, Valazquez FR. Indirect estimation of thyroid hormone-binding proteins to calculate free thyroxine index comparison of iionisotopic methods that use labeled thyroxine ( T-Uptake ). Cfin Chem 1995 41 41-7. 

Fig. 1. Schematic representation of the pathways of the biosynthesis of the thyroid hormones. The sites of biosynthetic defects are also indicated. [L] = active iodide," MIT = monoiodotyrosine, DIT = diiodotyrosine, Ts — triiodothyronine, T = thyroxine, TBP = thyroid hormone-binding proteins.

Fig. 2. Schematic representation of the interrelationship between the secretions of the hypothalamus, anterior pituitary, and thyroid glands, TRH = thyrotropinreleasing hormone, TSH thyroid-stimulating hormone (thyrotropin), T4 = i-thy-roxine, Ts = 1-triiodothyronine, TBP = thyroid hormone-binding proteins.

Bartalena L, Farsetti A, Flink IL, Robbins J (1992) Effects of interleukin-6 on the expression of thyroid hormone-binding protein genes in cultured human hepatoblastoma-derived (Hep G2) cells. Mol Endocrinol 6 935-942... 

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