Rutledge Lecture 3 u2013 Thyroid Gland.docx - Rutledge...

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Rutledge Lecture 3 – Thyroid Gland Angela Rugledge angela.rutledge @lhsc.on.ca Thyroid Gland Located in neck, below larynx, in front of trachea 10-20 grams, butterfly shaped, connected by isthmus (may have residual central lobe) Right lobe is larger than lef Gland size is larger with iodine deficiency Thyroid cells are arranged as follicles or groups of cells around a central lumen (colloid space) Thyroid hormones are synthesized within the follicular cells and stored on the protein thyroglobulin in the colloid space for later secretion The enzyme thyroperoxidase (TPO) oxidizes iodine that comes into the thyroid gland so that it can be added onto the tyrosyl moieties to create the thyroid hormones Thyroid follicles are connected together by connective tissue within the C-cells are scattered. These cells synthesize the hormone calcitonin , which helps to prevent the circulating free calcium level from getting too high The thyroid hormone made in the greatest abundance by the thyroid gland is thyroxine (tetra- iodothyronine, T 4 ) The thyroid gland also produces some tri-iodothyronine (T 3 ) , which is about 10x more active than T 4 , but most of the body’s T 3 is produced in peripheral tissues by selective removal of one iodine from the 5’ position of T 4 by enzymes called deiodinases Throxine is an iodinated derivative of coupled tyrosine molecules Mono and di-iodotyrosine may combine to form T 3 and T 4 Upon their release from the follicular colloid of the gland, most T 4 and T 3 are bound to carrier proteins (thyroid-binding globulin, albumin, and pre-albumin) T 4 : less than 0.015% circulates as free hormone T 3 : less than 0.33% circulates as free hormone Only free hormones are biologically active Negative Feedback Loop Thyroid hormone synthesis is stimulated by thyroid stimulating hormone (TSH) which is secreted from the anterior pituitary in response to the hypothalamic hormone thyrotropin-releasing hormone (TRH) TSH secretion is in turn inhibited by high circulating levels of thyroid hormones, which exert negative feedback both on the hypothalamus to decrease TRH and directly on the pituitary to inhibit TSH

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