Thyroid gland_.pdf

12/11/23 Thyroid gland: Learning objective: de ne and describe an overview of thyroid hormone synthesis. Learning objective: explain the role of thyroid hormones in metabolism. Learning objective: explain the role of thyroid, hormones in growth and neuronal development. Learning objective: de ne and...

12/11/23 Thyroid gland: Learning objective: de ne and describe an overview of thyroid hormone synthesis. Learning objective: explain the role of thyroid hormones in metabolism. Learning objective: explain the role of thyroid, hormones in growth and neuronal development. Learning objective: de ne and describe a brief overview of T3/T4 de ciency in an adult/child. Thyroid histology: The lobes of the thyroid contain follicles. Bet ween these follicles are C cells which secrete calcitonin. Follicles consist of a layer of epithelium. Central cavities containing colloid. Major constituent of colloid is the large glycoprotein, thyroglobulin. Follicular cells are TSH dependent. Synthesis, storage and secretion of thyroid hormones: colloid Tyrosine and iodine are essential for the synthesis of thyroid hormones. Both are taken up by the blood. Tyrosine is synthesised by the body ( in the thyroglobulin ). Iodine is a dietary essential. Other endocrine glands secrete their hormone once produced, the thyroid gland stores considerable amounts of thyroid hormones in the colloid until needed. Steps in synthesis, storage and secretion of thyroid hormones: 1. Thyroglobulin produced by follicular cells and released into colloid in follicular lumen by exocytosis. 2. Iodide ( I-) uptake by the follicular cell from the blood, oxidised and transferred to colloid. 3. Attachments of iodine into tyrosine residues on thyroglobulin in colloid forming di and monoiodotyrosine ( DIT and MIT ). 4. Coupling process of the iodinated tyrosine to form T4 ( 2xDIT ) and T3 ( DIT + MIT ). 5. Secretion ( upon stimulation ) of T4/T3 occurs by endocytosis. Fusion with a lysosome, uncoupling of T4 and T3 and diffusion out of the follicular cell into the blood and onto peripheral tissue. A majority of the T4 that is secreted is subsequently converted to T3. T3 is formed from monodeiodination of T4 in the thyroid and in peripheral tissues. T3 is four times more potent in its biological form than T4 and is the major hormone that will interact with target cells. TSH: mechanism of action to stimulate thyroid hormone release: TSH receptors are members of the large family of G-protein coupled receptors. The major second messenger is cAMP. Physiological effects of thyroid hormones: Increase in metabolic activities. Increase in oxygen consumption to most metabolically active tissues. Since there has been an increase in metabolism, this results in an increase in heat production. The thyroid hormone effects are calorgenic. Heat producing Sympathomimetic effect: Sympathomimetic: any action similar to one produced by the sympathetic nervous system. Thyroid hormone increases target cell responsiveness to catecholamines. Thyroid hormones are permissive and increased production of speci c catecholamine target cell receptors. The cardiovascular system: Increases the heart responsiveness to circulating catecholamines. Increases heart rate and force of contraction which leads to an increase in cardiac output. In response to heat load, peripheral vasodilation occurs to eliminate generation of extra heat.