Unit Fourteen: Endocrinology and Reproduction

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-Graves 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