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


• Iodine is Required for the Formation of Thyroxine

a. Iodine in the form of iodides; 1.0 mg/wkb. 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


• 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


• Iodide Pump-the Sodium-Iodide Symporter(Iodide Trapping)

a. Transport of iodine from the bloodb. Formation and secretion of thyroglobulin by the

thyroid cellsc. Oxidation of the iodide iond. Iodination of tyrosine and formation of the thyroid

hormone (organification of thyroid)


Fig. 76.3 Chemistry of thyroxine and triiodothyronine formation


e. Storage of thyroglobulin-enough is stored to last the body for 2-3 months

f. Release of throxine and triiodithyronine-cleaved fromthe thyroglobulin and then released into the blood

g. Daily rate of secretion; 93% is normally thyroxine and7% triiodothyronine. However, about ½ of the thyroxine is slowly deiodinated to form the T3 so the tissues get mainly T3


h. Thyroxine and triodothyronine are transported boundto plasma proteins

i. Because of the high affinity to the plasma proteins, the hormone is released very slowly

j. Thyroid hormones have slow onset and long durationof action


Fig. 76.4 Approximate prolonged effect on the basal metabolic rate caused by administering a single large dose of thyroxine

Physiological Functions of the Thyroid Hormones

• Thyroid Hormones Increase the Transcription ofLarge Numbers of Genes

a. Most of the thyroxine secreted by the thyroid is converted to triiodothyronine (T3)

b. Thyroid hormones activate nuclear receptors


Fig. 76.5 Thyroid hormone activation of target cells


• Thyroid Hormones Increase Metabolic Activity-

(increase the BMR 60-100x)

a. Thyroid hormones increase the number and activity of mitochondria

b. Increase the active transport of ions through thecell membrane (sodium and potassium)


• Thyroid Hormones Effect on Growth

a. Promote the growth and development of the brainduring fetal life and first years of postnatal life

b. Deficiency will retard growth during growing years


• Effect on Specific Bodily Mechanisms

a. Stimulation of cbh metabolism-rapid uptake of glucose, enhanced glycolysis, enhanced gluconeo-genesis, increased rate of absorption, increasedinsulin secretion

b. Stimulation of fat metabolism-lipids are mobilizedrapidly decreasing fat stores, increases free fattyacid concentration in plasma, and accelerates theoxidation of free fatty acids in cells


• Effect on Specific Bodily Mechanisms

c. Increased thyroid hormone decreases the concentrations of cholesterol, phospholipids, andtriglycerides in plasma and vice versa; increasesthe cholesterol secretion in bile

• Increased Requirement for Vitamins

• Increased Metabolic Rate

• Decreased Body Weight


Fig. 76.6 Approximate relation of daily rate of thyroid hormone (T4 and T3) to the basal metabolic rate


• Effect on the Cardiovascular System

a. Increased blood flow and cardiac outputb. Increased heart ratec. Increased heart strengthd. Normal arterial pressure

• Increased Respiration• Increased Gastrointestinal Motility• Excitatory Effects on the CNS• Muscles React With Vigor


• 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

Regulation of Thyroid Hormone Secretion

• TSH (Anterior Pituitary) Increases Thyroid Secretion

a. Increased proteolysis of the thyroglobulinb. Increased activity of the iodide pumpc. Increased iodination tyrosined. Increased size and secretory activity of the

thyroid cells

e. Increased number of thyroid cells

• Cyclic AMP Mediates the Stimulatory Effect of TSH-acting as a second messenger system


• 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


• Diseases of the Thyroid

a. Hyperthyroidism-Grave’s Disease, toxic goiter

b. Symptoms of hyperthyroidism

1) High state of excitability2) Intolerance to heat3) Mild to extreme weight loss4) Varying degrees of diarrhea5) Muscle weakness6) Extreme fatigue7) Tremor of the hands8) Exophthalmos


• Diseases of the Thyroid

c. Hypothyroidism-endemic colloidal goiter caused byiodine deficiency

d. Symptoms of hypothyroidism

1) Myxedema2) Cretinism

Unit Fourteen: Endocrinology and Reproduction

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