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
Synthesis and Secretion of the Thyroid Metabolic Hormones
• Iodine is Required for the Formation of Thyroxine
a. Iodine in the form of iodides; 1.0 mg/wk
b. 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
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
Synthesis and Secretion of the Thyroid Metabolic Hormones
• Iodide Pump-the Sodium-Iodide Symporter
(Iodide Trapping)
a. Transport of iodine from the blood
b. Formation and secretion of thyroglobulin by the
thyroid cells
c. Oxidation of the iodide ion
d. Iodination of tyrosine and formation of the thyroid
hormone (organification of thyroid)
Synthesis and Secretion of the Thyroid Metabolic Hormones
Fig. 76.3 Chemistry of thyroxine and
triiodothyronine formation
Synthesis and Secretion of the Thyroid Metabolic Hormones
e. Storage of thyroglobulin-enough is stored to last the
body for 2-3 months
f. Release of throxine and triiodithyronine-cleaved from
the thyroglobulin and then released into the blood
g. 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
Synthesis and Secretion of the Thyroid Metabolic Hormones
h. Thyroxine and triodothyronine are transported bound
to 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 duration
of action
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
Physiological Functions of the Thyroid Hormones
• Thyroid Hormones Increase the Transcription of
Large Numbers of Genes
a. Most of the thyroxine secreted by the thyroid is
converted to triiodothyronine (T3)
b. Thyroid hormones activate nuclear receptors
Physiological Functions of the Thyroid Hormones
Fig. 76.5 Thyroid hormone
activation of target cells
Physiological Functions of the Thyroid Hormones
• 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 the
cell membrane (sodium and potassium)
Physiological Functions of the Thyroid Hormones
• Thyroid Hormones Effect on Growth
a. Promote the growth and development of the brain
during fetal life and first years of postnatal life
b. Deficiency will retard growth during growing
years
Physiological Functions of the Thyroid Hormones
• Effect on Specific Bodily Mechanisms
a. Stimulation of cbh metabolism-rapid uptake of
glucose, enhanced glycolysis, enhanced gluconeogenesis, increased rate of absorption, increased
insulin secretion
b. 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
Physiological Functions of the Thyroid Hormones
• Effect on Specific Bodily Mechanisms
c. 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
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
Physiological Functions of the Thyroid Hormones
• Effect on the Cardiovascular System
a.
b.
c.
d.
•
•
•
•
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
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
Regulation of Thyroid Hormone Secretion
• TSH (Anterior Pituitary) Increases Thyroid Secretion
a.
b.
c.
d.
Increased proteolysis of the thyroglobulin
Increased activity of the iodide pump
Increased iodination tyrosine
Increased size and secretory activity of the thyroid
cells
e. Increased number of thyroid cells
•
Cyclic AMP Mediates the Stimulatory Effect of TSHacting as a second messenger system
Regulation of Thyroid Hormone Secretion
• Secretion of TSH is Regulated by ThyrotropinReleasing Hormone from the Hypothalamus
• Feedback Effect of Thyroid Hormone to Decrease
the Secretion of TSH
Fig. 76.7 Regulation of thyroid secretion
Regulation of Thyroid Hormone Secretion
• Diseases of the Thyroid
a. Hyperthyroidism-Grave’s Disease, toxic goiter
b. Symptoms of hyperthyroidism
1)
2)
3)
4)
5)
6)
7)
8)
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
Regulation of Thyroid Hormone Secretion
• Diseases of the Thyroid
c. Hypothyroidism-endemic colloidal goiter caused by
iodine deficiency
a. Symptoms of hypothyroidism
1) Myxedema
2) Cretinism