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Unit Fourteen: Endocrinology and Reproduction

Unit Fourteen: Endocrinology and Reproduction. Chapter 76: Thyroid Metabolic Hormones. Guyton and Hall, Textbook of Medical Physiology, 12 edition. Synthesis and Secretion of the Thyroid Metabolic Hormones. Physiologic Anatomy of the Thyroid. Fig. 76.1.

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Unit Fourteen: Endocrinology and Reproduction

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  1. Unit Fourteen: Endocrinology and Reproduction Chapter 76: Thyroid Metabolic Hormones Guyton and Hall, Textbook of Medical Physiology, 12 edition

  2. Synthesis and Secretion of the Thyroid Metabolic Hormones • Physiologic Anatomy of the Thyroid Fig. 76.1

  3. Synthesis and Secretion of the Thyroid Metabolic Hormones • Iodine is Required for the Formation of Thyroxine • Iodine in the form of iodides; 1.0 mg/wk • Of the iodide absorbed from the intestine, 80% is • rapidly excreted by the kidneys, and 20% is • selectively removed by cells of the thyroid gland

  4. Synthesis and Secretion of the Thyroid Metabolic Hormones • Iodide Pump-the Sodium-Iodide Symporter • (Iodide Trapping) Fir. 76.2 Thyroid cellular mechanisms for iodine transport, thyroxine and triiodothyronine formation, and thyroxine and triiodithyronine release into the blood

  5. Synthesis and Secretion of the Thyroid Metabolic Hormones • Iodide Pump-the Sodium-Iodide Symporter • (Iodide Trapping) • Transport of iodine from the blood • Formation and secretion of thyroglobulin by the • thyroid cells • Oxidation of the iodide ion • Iodination of tyrosine and formation of the thyroid • hormone (organification of thyroid)

  6. Synthesis and Secretion of the Thyroid Metabolic Hormones Fig. 76.3 Chemistry of thyroxine and triiodothyronine formation

  7. Synthesis and Secretion of the Thyroid Metabolic Hormones Storage of thyroglobulin-enough is stored to last the body for 2-3 months Release of throxine and triiodithyronine-cleaved from the thyroglobulin and then released into the blood Daily rate of secretion; 93% is normally thyroxine and 7% triiodothyronine. However, about ½ of the thyroxine is slowly deiodinated to form the T3 so the tissues get mainly T3

  8. Synthesis and Secretion of the Thyroid Metabolic Hormones Thyroxine and triodothyronine are transported bound to plasma proteins Because of the high affinity to the plasma proteins, the hormone is released very slowly Thyroid hormones have slow onset and long duration of action

  9. Synthesis and Secretion of the Thyroid Metabolic Hormones Fig. 76.4 Approximate prolonged effect on the basal metabolic rate caused by administering a single large dose of thyroxine

  10. Physiological Functions of the Thyroid Hormones • Thyroid Hormones Increase the Transcription of • Large Numbers of Genes • Most of the thyroxine secreted by the thyroid is • converted to triiodothyronine (T3) • Thyroid hormones activate nuclear receptors

  11. Physiological Functions of the Thyroid Hormones Fig. 76.5 Thyroid hormone activation of target cells

  12. Physiological Functions of the Thyroid Hormones • Thyroid Hormones Increase Metabolic Activity- • (increase the BMR 60-100x) • Thyroid hormones increase the number and activity of mitochondria • Increase the active transport of ions through the • cell membrane (sodium and potassium)

  13. Physiological Functions of the Thyroid Hormones • Thyroid Hormones Effect on Growth • Promote the growth and development of the brain • during fetal life and first years of postnatal life • Deficiency will retard growth during growing • years

  14. Physiological Functions of the Thyroid Hormones • Effect on Specific Bodily Mechanisms • Stimulation of cbh metabolism-rapid uptake of • glucose, enhanced glycolysis, enhanced gluconeo- • genesis, increased rate of absorption, increased • insulin secretion • Stimulation of fat metabolism-lipids are mobilized • rapidly decreasing fat stores, increases free fatty • acid concentration in plasma, and accelerates the • oxidation of free fatty acids in cells

  15. Physiological Functions of the Thyroid Hormones • Effect on Specific Bodily Mechanisms • Increased thyroid hormone decreases the • concentrations of cholesterol, phospholipids, and • triglycerides in plasma and vice versa; increases • the cholesterol secretion in bile • Increased Requirement for Vitamins • Increased Metabolic Rate • Decreased Body Weight

  16. Physiological Functions of the Thyroid Hormones Fig. 76.6 Approximate relation of daily rate of thyroid hormone (T4 and T3) to the basal metabolic rate

  17. Physiological Functions of the Thyroid Hormones • Effect on the Cardiovascular System • Increased blood flow and cardiac output • Increased heart rate • Increased heart strength • Normal arterial pressure • Increased Respiration • Increased Gastrointestinal Motility • Excitatory Effects on the CNS • Muscles React With Vigor

  18. Physiological Functions of the Thyroid Hormones • Muscle Tremors with Hyperthyroidism • Difficulty in Sleeping and Constant Tiredness • With Hyperthyroidism • Increased Thyroid Hormone Increases the • Secretion of Several Other Endocrine Glands • Needs to be Normal for Normal Sexual Function

  19. Regulation of Thyroid Hormone Secretion • TSH (Anterior Pituitary) Increases Thyroid Secretion • Increased proteolysis of the thyroglobulin • Increased activity of the iodide pump • Increased iodination tyrosine • Increased size and secretory activity of the thyroid • cells • Increased number of thyroid cells • Cyclic AMP Mediates the Stimulatory Effect of TSH- • acting as a second messenger system

  20. Regulation of Thyroid Hormone Secretion • Secretion of TSH is Regulated by Thyrotropin- • Releasing Hormone from the Hypothalamus • Feedback Effect of Thyroid Hormone to Decrease • the Secretion of TSH Fig. 76.7 Regulation of thyroid secretion

  21. Regulation of Thyroid Hormone Secretion • Diseases of the Thyroid • Hyperthyroidism-Grave’s Disease, toxic goiter • Symptoms of hyperthyroidism • High state of excitability • Intolerance to heat • Mild to extreme weight loss • Varying degrees of diarrhea • Muscle weakness • Extreme fatigue • Tremor of the hands • Exophthalmos

  22. Regulation of Thyroid Hormone Secretion • Diseases of the Thyroid • Hypothyroidism-endemic colloidal goiter caused by • iodine deficiency • Symptoms of hypothyroidism • Myxedema • Cretinism

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