...ine e.g.,Epinephrine,Norepinephrine and thyroid Hormones.3.PeptideProtein HormonesThese are either large proteins as small as medium size Peptides e.g.Insulin,glcogen,Parathormone,Cacitonine,Pituit
ary Hormones. Keyordshormones,thyroid,hypotalamus,insulin IntroductionHormones are substances produced by Endocrine cells,realised into and transported by the circulatory system to their target organs,here they exert their effects.Paracrin function,in contrast to endocrine action,involves the release and action of substances on neighbouring cells.Autocrin function involves the release of an effects that then acts upon the cell that released the substance.The latter,to functions,paracrine and autocrine,do not involve transport by the circulatory system.Hormones are classified as lipid soluble and ater soluble.The lipid soluble hormones includesteroid hormones,the thyroid hormones and 1,25-dihydroxycholecalciferal,the ater soluble hormones include cathecol amines,small peptides,polypeptides,proteins and glycoproteins.Thyroid Hormones The principal hormones secreted by the follicular cells of thyroid are 1.TyroxineT4 2.Tri-iodo-thyronineT3 3.Reverse T3 Chemistry of Thyroid HormonesThe hormones T4,T3 and reverse T3 are iodinated aminoacid tyrosine.The iodine in in tyrosine accounts for 80 of the organically-bound iodine in thyroid venous blood.Small amounts of reverse tri-iodo-thyronine monoiodotyrosineMIT and other compound are also liberated.Biosynthesis of Thyroid HormonesTo ra materialssubstances required by thyroid gland to synthesise the thyroid hormones-Thyroglobulin-IodineThyroid hormones are synthesised by the iodination of tyrosine residues of a large protein called thyroglobulin,hich is a dimmer glycoprotein ith a molecular eight 660 000.The receptor tyrosine molecules contains 115 tyrosine residues.Thyroid acinar cells have three functions1.they synthesise thyroglobulin and stores as colloid in follicles2.They collect and transport T2 for synthesis of the hormones in the colloid3.They remove T3 and T4 from thyroglobulin secreting the hormones into the circulation. Thyroglobulin is translated by the polysomes on the granular endoplasmatic reticulum of thyroid cells.The glycosylation of the molecule starts in the smooth endoplasmatic reticulum ith the incorporation monnose and the other sugars as N-acetylglucosamine,Gal,fructose and sialic acid.The glicoproteins formed as above one packaged into small vesicles.The vesicles them move toards the apical plasma membrane and fuse ith it,releasing their contents into the colloid of the thyroid follicles.The other substrat required for thyroid hormone synthesis is iodine.Vegetables and and fruits gron and obtained from sea-shore and also sea fish are rich in iodine.Vegetables and fruits in hilly regions lock iodine.Ingested dietary iodine is converted to iodide and absorbed from the gut.Of a total 50 mg of iodide in the body about 10 to 15 mg are in thyroids.The normal daily intake of iodide is 100 to 200mg . Minimum requirement of iodine is 25 mg . this iodide is absorbed mainly from small intestine and is transported in plasma in loose attachment to protein can alsa be absorbed from Lungs , other mucous membranes and skin. About 23 of the ingested in excreted by the kidneys , the remaining 13 is taken up by the thyroid gland for synthesis of thyroid hormones. Thyroid stimulating hormone of anterior pituitary gland stimulates iodine uptake by the thyroid gland.Metabolism of thyroid hormonesLiver plays an important role in the regulation of concentration and biologocal activity of thyroid hormones in blood.De-iodination of hormones occurs in liver.Iodine is excreted as inorganic iodide in the bile and the urine.Conjugation of iodothyronins ith glucuronic acid takes places in liver resulting to glucuronides being excreted in bile.Action of Thyroid HormonesThe target cells include hepatocytes,renal cells,cardiac cells,neurons,lymphocytes and pituitary somatotrophs.The biological effects of thyroid hormones are -calorigenic effect thyroid hormones considerably enhance O2 consuption and oxygen coefficient in many tissues except brain ,testes and spleen and increase the heat production and BMR of the organism .-groth and differentiation effects thyroide hormones act synergistically ith groth hormone to promote N retention,protein synthesis positive N balance,body groth and tissue diferention,by and enhancing the transcription of genes for tissue proteins.-effects on carbohidrate metabolismthyroid hormones raise the blood sugar,reduce glucose tolerance and increase glucose utilisation.-effects on lipid metabolismthyroid hormones potentiate the lipolytic action of adrenaline by up-regulating the beta-adrenergic receptors on the adipocyte membrane.-effects on protein metabolismthyroid hormones produce protein anabolic effects in moderate doses,but inhibit protein synthesis and increase blood amino acid level and urinary NPN in large doses.Hypothyroidism causes Cretinism in children and Myxoedema in adults.Groth in children is retarded and the child is mentally defective.He has coarse scanty hair and thick yelloish scaly skin.Cretinism occurs in areas here goitre is prevalent.In Myxoedema occurs the puffiness of the face and hands in adults,the retention of ater and NACl in the body,body temperature and pulse rate are subnormal,body eight is increased due to deposition of fat and retention of ater,mental faculties are retarded and blood cholesterol and lipid levers are increased. Hashimotos deseaseThyroglobulin escapes from the cells of the gland and excites the production of antibodies hich produce reactions ith thyroid.Fibroses of the thyroid tissue develop leading to complete loss of thyroid function.Severe hyperthyroidism leads to toxic goiter in hich the patient complains of nervousness,restlessness,tiredness,undue seating,breathless on exertion,tachycardia and palpitations,the subject cannot tolerate arm climate but can tolerate severe cold climate.Hyperthyroidism is characterised bynervousness,eight loss,hyperphagia,heat intolerance,increased pulse pressure,a fine tremor of the outstretched fingers.Hypothalamic hormonesHypothalamus secrets several peptide hormones hich regulate secretion of the anterior pituitary hormones. These hypophysiotropic hormones include-thyrotropine releasing hormone-corticotropin releasing hormone-gonadotropin-groth-somatostatin-prolactin release-vasoactive intestinal peptide.They are synthesised by hypotalamic peptidergic neurones.Groth hormone releasing is a peptide made of 44 aminoacids. Vasoactive intestinal peptide stimulates the lactotroph and corticotrophin cell of anterior pituitary to secrete respectively prolactin and corticotrophin.Thyrotropin releasing hormone is a natural tripeptide. It stimulates the transcription of the thyrotrophyne gene in pituitary thyrotropin cells to increase the synthesis of thyrotropin.Gonadotropi...
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