THYROID HORMONES

THYROID HORMONES - Hypothalamic Pituitary-Thyroidal Axis The hypothalamus releases THYROID RELEASING HORMONE(TRH via the portal system TRH binds to

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Hypothalamic- Pituitary -Thyroidal Axis The hypothalamus releases THYROID RELEASING HORMONE ( TRH) via the portal system TRH binds to TRH receptors on the thyrotrophs in the anterior pituitary to stimulate the thyrotrophs to produce THYROID STIMULATING HORMONE ( TSH) ( also known as thyrotropin) TSH binds to TSH receptors on follicle cells in the thyroid gland to stimulate the production of the 2 THYROID HORMONES: tetraiodothyronine(T4) triiodothyronine (T3)
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Hypothalamic-Pituitary-Thyroid Axis
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The Thyroid Gland Location : anterior neck region just below the larynx Structure : Butterfly-shaped – 2 lobes connected by the isthmus. Each lobe is 2.5cm-4cm long; 1.5cm-2cm wide; 1cm-1.5cm thick.
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The Thyroid Gland Thyroid gland is the largest endocrine gland in the body Internally, the thyroid gland is composed of spherical hollow structures called follicles - follicles contain a thick amber-colored fluid called COLLOID (# 2 in the diagram below) The walls of the follicles are formed by the follicle cells (#1) The follicle cells secrete a large glycoprotein called THYROGLOBULIN into the colloid. Thyroglobulin is composed of 5496 amino acids of which 140 are of the amino acid TYROSINE
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Biosynthesis of the Thyroid Hormones 1. Iodide trapping – active transport of iodide from blood by the thyroid follicle cells 2. Oxidation of iodide into Iodine and iodine is released into the colloid catalyzed by enzyme, thyroperoxidase (TPO) 3. Iodination of thyroglobulin to form the 2 types of IODOTYROSINES – MONOIODOTYROSINE ( MIT) and DIIODOTYROSINE (DIT ). MIT - produced by the attachment of one iodine to a tyrosine in the thyroglobulin DIT – produced by the attachment of two iodines to tyrosine in the thyroglobulin. Under normal physiological conditions, more DITs are formed 4. Organification - coupling of the iodotyrosines to form the two types of IODOTHYRONINES – TETRAIODOTHYRONINE (T4 ) ( also known as thyroxine ) and TRIIODOTHYRONINE (T3) DIT + DIT = T4 under normal physiological conditions, more T4 is produced MIT + DIT = T3 5. Endocytosis of thyroglobulin - thyroglobulin containing T4 and T3 is taken back into the follicle cells via endocytosis 6. Proteolysis of Thyroglobulin -Lysosomal digestion of the thyroglobulin to release T4 and T3 into the bloodstream; more T4 (100 nmol/L) is released than T3 (5 nmol/L)
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Factors that affect the release of the Thyroid Hormones Stimulatory factors: 1. TRH 2. TSH 3. Exposure to cold 4. Decreased activity of Type 2 deiodinase = increased TSH 5. Estrogens Inhibitory factors: 1. Increased blood levels of T4/T3 via NFB 2. Increased activity of Type 2 deiodinase = decreased TSH 3. *IODIDE EXCESS* - Wolff-Chaikoff effect 4. Dopamine (suppresses TSH)
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This document was uploaded on 10/27/2011 for the course BSCI 447 at Maryland.

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THYROID HORMONES - Hypothalamic Pituitary-Thyroidal Axis The hypothalamus releases THYROID RELEASING HORMONE(TRH via the portal system TRH binds to

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