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14-September-2008 18:02:48 - Triiodothyronine Triiodothyronine IUPAC name 2S-2-amino-3- 4-4-hydroxy-3-iodo-phenoxy- 3,5-diiodo-phenylpropanoic acid Other names triiodothyronine T3 3,3',5-triiodo-L-thyronine Identifiers CAS number 6893-02-3 SMILES NC@@HCc1ccIcOc2cccOcIc2 cIc1CO=O Properties Molecular formula C15H12I3NO4 Molar mass 650.9776 g mol-1 Except where noted otherwise, data are given for materials in their standard state at 25 °C, 100 kPa Infobox references Triiodothyronine, C15H12I3NO4, also known as T3, is a thyroid hormone. Thyroid-stimulating hormone TSH activates the production of tetraiodothyronine T4 and T3. This process is under regulation. In the hypothalamus, T4 is converted to T3. TSH is inhibited mainly by T3. The thyroid gland releases greater amounts of T4 than T3, so plasma concentration of T4 are 40-fold higher than those T3. Most the circulating T3 is formed peripherally by deiodination of T4 85%, a process that involves the removal of iodine from carbon 5 on the outer ring of T4. Thus, T4 acts as prohormone for T3. This thyroid hormone is similar to thyroxine but with one fewer iodine atom per molecule. In addition, T3 exhibits greater activity and is produced in smaller quantity. It is the most powerful thyroid hormone, and affects almost every process in the body, including body temperature, growth, and heart rate. Contents 1 Production of T3 2 Transport of Triiodothyronine 3 Mechanism of Action 4 Effects of T3 5 The effects of abnormal thyroid functions Production of T3 T3 is metabolically active hormone that is produced from T4. T4 is deiodinated by two deiodinases to produce the active triiodothyronine: 1. Type I present within the liver and accounts for 80% of the deiodination of T4 2. Type II present within the pituitary. T4 is synthesised in the thyroid gland follicular cells as follows. 1. The Na+/I- symporter transports two sodium ions across the basement membrane of the follicular cells along with an iodine ion. This is secondary active transporter that utilises the concentration gradient of Na+ to move I- against its concentration gradient. 2. I- is moved across the apical membranae into the colloid of the follicle. 3. Thyroperoxidase oxidises two I- to form I2. Iodide is non-reactive and only the more reactive iodine is required for the next step. 4. The thyroperoxidase iodinates the tyrosyl residues of the thyroglobulin within the colloid. The thyroglobulin was synthesis in the ER of the follicular cell and secreted into the colloid. 5. Thyroid stimulating hormone TSH released from the pituitary gland binds the TSH receptor a Gs protein coupled receptor on the basolateral membrane of the cell and stimulates the endocytosis of the colloid. 6. The endosytosed vesicles fuse with the lysosomes of the follicular cell. The lysosomal enzymes cleave the T4 from the iodinated thyroglobulin. 7. These vesicles are then exocytosed releasing the thyroid hormones. In the follicular lumen, tyrosine residues become iodinated. This reaction requires hydrogen peroxide. Iodine bonds carbon 3 or carbon 5 of tyrosine residues of thyroglobulin in a process called organification of iodine. The iodination of specific tyrosines yields monoiodotyrosine MIT and diiodotyrosine DIT. One MIT and one DIT are enzymatically coupled to form T3. The enzyme is thyroid peroxidase. Synthesis Synthesis Transport of Triiodothyronine The T3 and T4 bind to nuclear receptors, thyroid receptors. However, T3 and T4 are not very lipophilic and as a result, are unable to pass through the phospholipid bilayers. They therefore have specific transport proteins on the cell membranes of the effector organs which allow the T3 and T4 to pass into the cells. The thyroid receptors bind to response elements in gene promoters and thus enabling them to activate or inhibit transcription. The sensitivity of a tissue to T3 is modulated through the thyroid receptors. Mechanism of Action T3 and T4 are carried in the blood bound to plasma proteins. This has the effect of increasing the half life of the hormone and decreasing the rate at which it is taken up by peripheral tissues. There are three main proteins that the two hormones are bound to. Thyronine binding globulin TBG is a gylcoprotein that has a higher affinity for T4 than for T3. The second plasma protein to which the hormone bind is transthyretin which has a higher affinity for T3 than for T4. Both hormones bind with a low affinity to albumin, but due to the large availability of albumin it has a high capacity. Effects of T3 T3 increases the basal metabolic rate and thus increases the body's oxygen and energy consumption. The basal metabolic rate is the minimal caloric requirement needed to sustain life in a resting individual. T3 acts on the majority of tissues within the body, with a few exceptions including the brain, spleen and testis. It increases the production of the Na+/K+ -ATPase and in general increases the turnover of different endogenous macromolecules by increasing their synthesis and degradation. Protein T3 stimulates the production of RNA Polymerase I and II and therefore increases the rate of protein synthesis. It also increases the rate of protein degradation and in excess the rate of protein degradation exceeds the rate of protein synthesis. In such situations the body may go into negative ion balance. Glucose T3 potentiates the effects of the β-adrenergic receptors on the metabolism of glucose. It therefore increases the rate of glycogen breakdown and glucose synthesis in gluconeogenesis. It also potentiates the effects of insulin, which have opposing effects. Lipids T3 stimulates the breakdown of cholesterol and increases the number of LDL receptors, therefore increasing the rate of lipolysis. T3 also affects the cardiovascular system. It increases the cardiac output by increasing the heart rate and force of contraction. This results in increased systolic blood pressure and decreased diastolic blood pressure. The latter two effects act to produce the typical bounding pulse seen in hyperthyroidism. T3 also has profound effect upon the developing embryo and infants. It affects the lungs and influences the postnatal growth of the central nervous system. It stimulates the production of myelin, neurotransmitters and axon growth. It is also important in the linear growth of bones. The effects of abnormal thyroid functions Goitre Goitre is the swelling of the thyroid gland. It is often associated with iodine deficiency. The lack of iodine decreases the production of the thyroid hormones T3 and T4. These hormones usually act on the pituitary gland to decrease the release of thyroid stimulating hormone TSH by negative feedback. Lack of this feedback causes the systemic levels of TSH to increase. One of the actions of TSH aside from stimulating the release of the thyroid hormones is to stimulate the growth of the thyroid gland. However, usually the enlarged thyroid will then act normally to trap sufficient iodine and thus the levels of T3 and T4 are normal. Goitre may also be a result of Grave's disease or of a tumour. Hyperthyroidism High levels of T3 . The symptoms of hyperthyroidism include: Raised basal metabolic rate Bounding pulse Heat intolerance Weight loss often accompanied by increased appetite Increased sympathetic drive Eye protrusion Hyperthryoidism may be caused by Grave's disease, an autoimmune disease where immunoglobulins that resemble TSH cause constitutive release of high levels of the thyroid hormones. On the other hand it may be due to a tumour of the thyroid gland. Hypothyroidism Low levels of T3 If this occurs during childhood it can result in gross deficiencies of myelination of the central nervous system neurons and stunting of growth due to decreased growth of the long bones. Hypothyroidism in the adult is known as myxedema, a condition where a reduced metabolism, slow mentation, hypothermia and constipation are seen due the lack of gut motility stimulated by T3. A cause of hypothyroidism is thyroid hormone deficiency, a genetic defect that reduces the hormone binding. This article about an organic compound is a stub. v d e Endocrine system: hormones/endocrine glands Peptide hormones, Steroid hormones Hypothalamic-pituitary Hypothalamus: TRH, CRH , GnRH, GHRH, somatostatin, dopamine - Posterior pituitary: vasopressin, oxytocin - Anterior pituitary: α FSH, LH, TSH, GH, prolactin, POMC ACTH, MSH, endorphins, lipotropin Adrenal axis Adrenal medulla: epinephrine, norepinephrine - Adrenal cortex: aldosterone, cortisol, DHEA Thyroid axis Thyroid: thyroid hormone T3 and T4 - calcitonin - Parathyroid: PTH Gonadal axis Testis: testosterone, AMH, inhibin - Ovary: estradiol, progesterone, inhibin/activin, relaxin pregnancy Other end. glands Pancreas: glucagon, insulin, somatostatin - Pineal gland: melatonin Non-end. glands Placenta: hCG, HPL, estrogen, progesterone - Kidney: renin, EPO, calcitriol, prostaglandin - Heart atrium: ANP - Stomach: gastrin, ghrelin - Duodenum: CCK, GIP, secretin, motilin, VIP - Ileum: enteroglucagon - Adipose tissue: leptin, adiponectin, resistin - Thymus: Thymosin - Thymopoietin - Thymulin - Skeleton: Osteocalcin - Liver/other: Insulin-like growth factor IGF-1, IGF-2 Target-derived NGF, BDNF, NT-3 Retrieved from http://en..org/wiki/Triiodothyronine Categories: Organic compound stubs | Iodinated tyrosine derivatives | Thyroid hormones Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages ÄŒesky Deutsch עברית Nederlands 日本語 Polski SlovenÅ¡Ä?ina Svenska This page was last modified on 18 August 2008, at 14:05
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