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 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 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 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 Thyroid hormone
activation of target cells

12. Physiological Functions of the Thyroid Hormones
Thyroid Hormones Increase Metabolic Activity-
(increase the BMR x)
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 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 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