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Thyroid Gland Prepared by Yousaf khan Anatomy of Thyroid Gland - The thyroid gland is situated in the neck in front of the larynx and trachea. - The thyroid gland is shaped like a butterfly, - just inferior to the larynx. - At the level of the 5th, 6th and 7th cervical and 1st thoracic vertebrae - Thyroid hormone secretion is controlled by TSH secreted by the anterior pituitary gland. Microscopic Structure of Thyroid Gland • • • • Gland consists of numerous follicles Walls of follicles are lined by cuboidal epithelium Each follicle contains “ colloid” The colloid consists of mainly iodinated thyroglobulin. • Parafollicular cells are present b/w the thyroid follicles. Histology of thyroid gland Hormones Secreted by thyroid Gland Three hormones are secreted 1. Thyroxine (T4) is secreded by follicular cells 2. Triiodothyronine (T3) is secreted by follicular cells 3. Calcitonin is secreted by parafollicular cells. Secretion of thyroid hormones • Thyroid hormones secretion is controlled by TSH secreted by anterior pituitary gland. • If TSH increase the thyroid secretions increase, • If TSH decrease thyroid hormones decrease. Elements required for synthesis • Elements required for synthesis of T3 and T4 A: Iodine B: Thyroglobulin Formation of thyroid hormones: About 90% of the hormone secreted by the thyroid gland is thryoxine and 10% is triiodothyronine. Difference b/w T3 and T4 • The function of these two hormones are qualitatively the same, but they differ in rapidity and intensity of action. • Triiodothyronine is about four times as potent as thyroxine, but it is present in the blood in much smaller quantities and persists for a much shorter time than does thyroxine Stages for the formation of T3 & T4 • Absorption of Ingested Iodides Iodine is absorbed from G.I.T into the blood in the form of iodides. A: Iodide Trapping B: Oxidation of Iodide ion C: Formation and secretion of Thyroglobulin D: Iodination of Tyrosine E: Storage of Thyroglobulin F: Release of T3 & T4 from the thyroid gland Why is Iodine Important in Thyroid Hormone Production? • Thyroid hormones are unique biological molecules in that they incorporate iodine in their structure. • Thus, adequate iodine intake (diet, water) is required for normal thyroid hormone production. • Major sources of iodine: - iodized salt - iodated bread - dairy products • Minimum requirement: 75 micrograms/day Transport of hormones • In the blood T4 & T3 combine with plasma protein, thyroid binding globulin (TBG) and are transport from one place to another in the blood. • TBG is more important not only for transport of thyroid hormones but also that it serves as reservoir of hormone. Effects of thyroid hormone on specific bodily mechanisms 1: Calorigenic Action (it maintains B.M.R) 2: Effects on carbohydrate metabolism (increase glucose absorption form GIT, Peripheral utilization of glocose, glycolysis in the liver, gluconeogenesis, and uptake of glucose by cells) 3: Effects on protein Metaboloism ( stimulate protein synthesis in ribosomes, normal range it increase protein anabolism while high conc. It causes protein catabolism, increase level of amino acid in the blood) Effects of thyroid hormone on specific bodily mechanisms 4: Effects on lipid Metabolism (increase mobilization of lipid from their sources, increase oxidation of free fatty acid, decrease cholesterol, phospholipids and triglyceride level of blood) 5: Effects on vitamin metabolism (T4 is necessary for conversion of Beta- carotene to vitamin A so in hypothyroidism blood carotene level increase to give a yellow color to skin. High B.M.R in hypothyroidism increase the requirement for vitamin B group, particularly thiamine (B2). Effects of thyroid hormone on specific bodily mechanisms 6: Effects on water and mineral metabolism ( promotes demineralization of bone, increase excretion of calcium and phosphates in the urine, it causes diuresis, increase K excretion) Specific effects • Effects on C.N.S 1: increased rapidity of cerebration 2: increased synaptic activity 3:hyperthyroidism is likely to cause extreme nervousness, anxiety and extreme worry. 4: in growing children decreased activity of thyroid hormone causes mental retardation • Effects on Heart 1: increase cardiac output 2:increase heart rate 3: increase strength of heart beat 4: increase peripheral blood flow • Effects on blood volume 1: slight increase in blood volume Specific effects • Effects on GIT 1: Increase the rate of absorption 2: increase the rate of secretion 3: increases the motility of GIT 4: increases the appetite • Effects on Respiration 1: causes increased utilization of O2 and formation of co2 2: increase pulmonary ventilation 3: increase the depth of respiration Specific effects • Effects of Gonads 1: essential for normal reproduction 2: secondary sexual characters do not develop with out this hormones 3: fertility is reduced in hypoactivity of hormone • Effects of mammary gland This promotes galactopoiesis ( formation of milk) Specific effects • Effects on muscle 1: muscular weakness occurs in both hypo and hyperthyroidism 2: slight increase in thyroid hormone usually makes the muscle react with vigor • Effects on sleep 1: hyperthyroid patients often have a constant feeling of tiredness due to excitable effects on synapses, it is difficult for patient to sleep. So in hyperthyroidism there is insomnia. THANK YOU