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A BRIEF OVERVIEW OF THE THYROID GLAND By April Stoll Anatomy of the Thyroid Gland Thyroid Basics: The main function of the thyroid is to control the body’s metabolism Its sole function is to make thyroid hormone This hormone has an effect on nearly all tissues of the body where it increases cellular activity The Thyroid Produces and Secretes 2 Metabolic Hormones The two principal hormones are: Thyroxine (T4 ) and triiodothyronine (T3) Required for homeostasis of all cells Influence cell differentiation, growth, and metabolism Stimulate protein production in the body’s tissues T4 is the most abundantly secreted, but T3 is considerably more active Considered the major metabolic hormones because they target virtually every tissue Thyroid Hormones Stimulate Metabolic Activities in Most Tissues Regulate the rate of overall body metabolism (specifically T3) T3 increases basal metabolic rate Increases body heat production Calorigenic effects T3 increases oxygen consumption by most peripheral tissues The Chemistry of Thyroid Hormones: Thyroid hormones are derivatives of the amino acid Tyrosine bound covalently to iodine These hormones are basically two tyrosine's linked together with the critical addition of iodine at the three of four positions on the aromatic rings T4/T3 Conversion Sites The liver is the major extrathyroidal T4 conversion site for production of T3 Some T4 to T3 conversion also occurs in the kidney and other tissues Thyroid-Stimulating Hormone (TSH) Also known as Thyrotropin Is secreted from cells, called thyrotrophs, in the anterior pituitary, which in turn is controlled by the hypothalamus Is the major regulator of the thyroid gland It Regulates thyroid hormone production, secretion, and growth Is regulated by the negative feedback action of T4 and T3 Its synthesis and release is stimulated by thyroid-releasing hormone (TRH) TSH-cAMP Thyroid-Stimulating Hormone-cyclic Adenosine Monophosphate (cAMP) Is the prime regulator of iodide uptake and concentration T3/T4 formation Induces the expression and activation of the 3 necessary genes that encode proteins involved in iodide uptake and thyroid hormone formation Sodium-iodide symporter (NIS) Thyroglobulin (Tg) Thyroperoxidase (TPO) Thyroid-Releasing Hormone (TRH) Is secreted by hypothalamic neurons Is only a tripeptide, with the basic sequence of amino acids being glutamic acid-histidine-proline Is inhibited by high blood levels of thyroid hormones in the negative feedback loop Is the major positive regulator of TSH secretions Hypothalamic-Pituitary-Thyroid Axis Negative Feedback Mechanism Thyroid Disorders Hypothyroidism Two common examples are: Iodine deficiency Primary thyroid disease Symptoms include: Lethargy Fatigue Cold-intolerance Weakness Hair loss Reproductive failure Hyperthyroidism The most common form is Graves Disease Is less common than Hypothyroidism Symptoms include: Nervousness insomnia high heart rate eye disease anxiety http://www.methodisthealth.com/endocrin/thygland.htm THE END ??? QUESTIONS ???