Thyroid hormones activation

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). 

Drugs with thyroid hormone activity, alone or with other therapeutic agents, have been used for the treatment of obesity. In euthyroid patients, doses within the range of daily hormonal requirements are ineffective for weight reduction. Larger doses may produce serious or even life-threatening manifestations of toxicity, particularly when given in association with sympathomimetic amines such as those used for their anorectic effects. 

The form of thyroid hormone active in binding its receptor is triiodothyronine (T3) rather than thyroxine (T4). 

Thyroid hormones can have a re-partitioning effect upon body composition or muscle-to-fat ratio. As example were the many athletes whose weight was 250 LBS but only 10% bodyfat when total daily circulating thyroid hormone levels were elevated 10-50%. This would be due to thyroid hormone activity inducing improved nutrient metabolization and cellular efficiency combined with other hormone synergy. Of course, this is what "Absolute Anabolic Phases" were all about. But those who read about" Frank N. Steroid" already know about this effect and how it was created. 

Thyroid hormone activity is determined predominantly by intracellular concentrations of (free) T3 in the tissues. This T3 bioavailability depends on 1) the secretion of T4 and T3 by the thyroid, 2) the conversion of T4 to T3 outside the thyroid, 3) the metabolic clearance of T3 and 4) exchange of T4 and T3 between plasma and tissues. It is the purpose of this chapter to review recent advancements in the study of the transport and metabolism of thyroid hormone. Especially the role of deiodination, conjugation and tissue uptake mechanisms will be emphasized in the regulation of thyroid hormone action. 

Kubo T, Matsumoto H, Shiraishi E, Nomachi M, Nemoto K, Hosoya K, Kaya K (2007) Selective separation of hydroxy polychlorinated biphenyls (HO-PCBs) by the structural recognition on the molecularly imprinted polymers Direct separation of the thyroid hormone active analogues from mixtures. Anal Chim Acta 589 180... 

Tien, E.S., Matsui, K., Moore, R. and Negishi, M. (2007) The nuclear receptor constitutively active/androstane receptor regulates type 1 deiodinase and thyroid hormone activity in the regenerating mouse liver. The Journal of Pharmacology and Experimental Therapeutics, 320, 307-313. 

T.SH is the feedback of thyroid hormone activity on the pituitary, and to a lesser extent on the hypothalamus. Other factors also play a role. There is a diurnal rhythm with the serum TSH peaking around midday when the concentration is approximately 30% higher than at midnight, when it isaliLsnadir. Insy.stemic illness the normal regulation of TSH.Tj and T, secretion, and the subsequent metabolism of the thyroid hormones, is disturbed. Increased amounts of T are converted to the biologically inactive reverse Tj, rather than to T,. The resultant reduction in thyroid hormone activity does not result in an increased serum TSH concentration. TSH secretion is also suppressed, and the secretion of T and T, by the thyroid gland is therefore decreased. 

Levothyroxine 21 Thyroid hormone receptor (agonist) Specific form of thyroid hormones, namely T4, which has 4 iodines Endogenous hormone family (thyroid hormones T4, T3, where T3 has 3 iodines) h Hypothyroidism Overextensison of thyroid hormone activity... 

Watanabe, M., Houten, S.M., Mataki, C., Christolfolete, M.A., Kim, B.W., Sato, H., Messaddeq, N., Harney, J.W., Ezaki, O., Kodama, T., et al. 2006. Bile acids induce energy expenditure by promoting intracellular thyroid hormone activation. Nature 439 484-489. 

K. R. Latham and Y. C. L. Tseng, Nuclear thyroid hormone activities... 

Thyroid hormone activity can be determined indirectly as the basal metabolic rate (q.v.). 

Kitamura S, Kato T, lida M et al (2005) Anti-thyroid hormonal activity of tetrabromobisphenol A, a flame retardant, and related compounds affinity to the mammalian thyroid hormone receptor, and effect on tadpole metamorphosis. Life Sci 18 1589-1601 Kuhn E, Arnet R, Kanzig A et al (2004) Bromierte flammschutzmittel in kunststoffprodukten des Schweizer marktes Swiss agency for the environment, forests and landscape. Bern, Switzerland. http //www.bafu.admin.ch/publikationen/publikation/00289/index.html lang=de. Accessed 01 May 2013... 

This remarkable ability of the thyroid to actively trap and store the iodine required creates a relatively steady state, with daily intake used to ensure full stores. T4, with a longer half-life, serves as a reservoir for conversion to the more active hormone, T3, with a much shorter half-life of 1 day. Target organs for thyroid hormone activity all play a role in the complex interplay between conversion of T4 to T3 deiodination, and metabolism of various other proteins involved with thyroid function. The liver, which is estimated to contain 30% of the extrathy-roidal T4, is responsible, through the activity of the liver cell enzyme, deiodinase, for ensuring adequate supply of T3 to peripheral tissues and degradation... 

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