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Endocrine Physiology THYROID GLAND Dr. Meg-angela Christi Amores Where is your thyroid gland located? • located immediately below the larynx on each side of and anterior to the trachea • normally weighing 15 to 20 grams in adults What are the hormones of the thyroid gland? • two major hormones: – Thyroxine(T4) and triiodothyronine(T3) – increase the metabolic rate of the body – Thyroid secretion is controlled primarily by thyroid-stimulating hormone (TSH) secreted by the anterior pituitary gland • also secretes calcitonin Synthesis and Secretion • 93% of metabolically active hormones secreted by the thyroid gland = THYROXINE (T4) • 7% = TRIIODOTHYRONINE (T3) • Triiodothyronine is about four times as potent as thyroxine Synthesis and Secretion • Physiologic Anatomy – Composed of large numbers of closed follicles – filled with a secretory substance called colloid and lined with cuboidal epithelial cells that secrete into the interior of the follicles Synthesis and Secretion • Thyroglobulin – major constituent of the colloid – contains the thyroid hormones within its molecule • Iodine – Required for formation of thyroxine – 50 milligrams of ingested iodine in the form of iodides are required each year, or about 1 mg/week • Transport of iodides into follicles (iodide pumping) • conversion to an oxidized form of iodine • Each molecule of thyroglogulin contains 70 tyrosine amino acids • Oxidized Iodine (I3)is capable of combining directly with the amino acid tyrosine • Oxidation is promoted by enzyme PEROXIDASE and accompanied with HYDROGEN PEROXIDE • binding of iodine with the thyroglobulin molecule is called organification of the thyroglobulin • successive stages of iodination of tyrosine and final formation of the two important thyroid hormones, thyroxine and triiodothyronine • Tyrosine is first iodized to monoiodotyrosine • and then to diiodotyrosine • Then, during the next few minutes, hours, and even days, more and more of the iodotyrosine residues become coupled with one another Transport of thyroid hormones • Thyroxine and Triiodothyronine Are Bound to Plasma Proteins – 99% of thyroid hormones are bound to: • thyroxine-binding globulin • thyroxine-binding prealbumin and albumin • Thyroid Hormones Have Slow Onset and Long Duration of Action • T4: long latency >2-3 days, maximumum in 10 – 12 days • T3: latent: 6 to 12 hours , maximum in 2 – 3 days Physiologic Function Physiologic Function • Thyroid Hormones Increase the Transcription of Large Numbers of Genes • Thyroid Hormones Increase Cellular Metabolic Activity • Effect on growth: • thyroid hormone is essential for the metamorphic change of the tadpole into the frog • promote growth and development of the brain during fetal life and for the first few years of postnatal life Physiologic Function • Effects on specific bodily mechanisms – Stimulation of Carbohydrate Metabolism – Stimulation of Fat Metabolism – Increased Requirement for Vitamins – Increased Basal Metabolic Rate – Decreased Body Weight Regulation of Thyroid hormone