Thyroid hormone storage

Synthesis, storage, and release of thyroid hormones by the thyroid gland are primarily regulated by the thyrotropin hormone, while the iodides necessary for their synthesis are usually present in consumed foods. 

BIOSYNTHESIS, STORAGE, SECRETION, AND METABOLISM OF THYROID HORMONES... 

Thyroxine (T4) and the more potent triiodothyronine (T3) are cleaved from a large precursor protein called thyroglob-ulin. Thyroglobulin exists as a dimer of two identical polypeptides (Mr 330,000). It is a storage protein for iodine and can be considered a prohormone of the circulating thyroid hormones. Thyroglobulin is secreted into the lumen of the thyroid gland, where specific residues are iodinated in... 

Thyroid hormones govern the body s metabolic rate. This means that the metabolism of nutrients and subsequent cellular utilization or storage rate is dependent upon blood circulatory thyroid hormone levels. Higher levels result in elevated over all metabolic rate providing that other metabolic factors are accommodated also. 

The thyroid gland is made up of multiple follicles that consist of a single layer of epithelial cells surrounding a lumen filled with colloid (thyroglobulin), the storage form of thyroid hormone. A diagram of the steps in thyroid hormone synthesis and secretion is shown in Figure 25.6. 

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). 

T3 has an amplifying or facilitatory effect on the lipolytic action of epinephrine on the fat cell. Yet thyroid hormone has a bipolar effect on lipid storage, because it increases the availability of glucose to the fat cell, which serves as a precursor for fatty acid and glycerol 3-phosphate synthesis. The major determinant of the rate of lipogenesis, however, is not T3, but rather the amount of glucose and insulin available to the adipocyte for triacylglycerol synthesis. 

Tg, the storage protein of thyroid hormones, is an autoantigen associated with autoimmune thyroid disease. It induces EAT, a mouse model for Hashimotos thyroiditis, which is the prevalent hypothyroid syndrome. 

Iodine deficiency results in lowered maternal circulating thyroid hormone concentrations, leading to reduction in placental transfer of thyroxine. The possibility of iodine storage by the placenta compensating for diminished dietary iodine intake has been suggested by Delange (2004), and placental iodine storage has been demonstrated (Smyth et al, 2006). 

Thyroglobulin is a glycoprotein of approximately 660 000 molecular weight which comprises the bulk of the colloid in the thyroid follicles. It serves as a storage vehicle for the two thyroid hormones thyroxine (80) and 3,3, 5-tri-iodothyronine (76) and provides a matrix for their synthesis. The molecule of thyroglobulin contains approximately 120 residues of L-tyrosine and under normal conditions some 10-25 of these are present in iodinated forms and up to five in the form of thyroxine ". ... 

Hormones—Many of the above changes in metabolism are ultimately due to some hormonal change. Most of the hormones direct the metabolic processes. The storage and release of glucose is hormone mediated. Protein synthesis is hormone mediated, as is the release of fatty acids from fats. The thyroid hormones can, on a long-term basis, raise the metabolic rate 100%. Conversely, undersecretion can cause the metabolic rate to fall 50% below normal. Other hormones control water and minerals, and still others control the digestive processes. 

K.V. Waite, G.F. Maberly, and C.J. Eastman, Storage conditions and stability of thyrotropin and thyroid hormones on filter paper, Clinical Chemistry 33(6) 853-855(1987). 

As the human body is able to store many minerals, deviations from the daily ration are balanced out over a given period of time. Minerals stored in the body include water, which is distributed throughout the whole body calcium, stored in the form of apatite in the bones (see p. 340) iodine, stored as thyroglobulin in the thyroid and iron, stored in the form of ferritin and hemosiderin in the bone marrow, spleen, and liver (see p. 286). The storage site for many trace elements is the liver. In many cases, the metabolism of minerals is regulated by hormones—for example, the uptake and excretion of H2O, Na, ... 

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