1. PHYSIOLOGY OF THE ENDOCRINE SYSTEM
Dr. Sajeda Al-Chalabi
Assist.Proff/Dept .of Physiology

2. The Thyroid Gland Lies near the thyroid cartilage of the larynx
Two lobes connected by an isthmus

4. The thyroid gland
Formation & secretion of thyroid hormones : The main hormones secreted by the thyroid are thyroxine(T4) &triiodothronine(T3) T3 is also formed in the peripheral tissues by deiodination of T4.both hormones are iodine containing amino acids.T3 is more active than T4. Iodied pump : The first stage in the formation of thyroid hormones is transport of iodieds from the blood into the thyroid glandular cells & follicles this is called iodied trapping , in normal gland the iodied pump concentrates the iodied to about 30 times its concentration in blood. the rate of iodide trapping is influenced by several factors , the most important is the concentration of TSH.

5. Thyroglobulin & chemistry of thyroxin & triiodothyronin formation
Thyroglobulin is a large glycoprotein molecule has a molecular weight of about 335,000.the molecule of thyroglobulin contains 70 tyrosine amino acids &they are the major substrates that combine with iodine to form thyroid hormones .thus thyroid hormones form within the thyroglobulin molecule .
Thyroglobulin is synthesized in the thyroid cells & secreted into the colloid . the first step in the formation of thyroid hormones is conversion of iodide ions to an oxidized form of iodine , this oxidation of iodine is promoted by the enzyme peroxidase . tyrosine is first iodized to monoiodotyrosine (MIT) this is next iodinated to form diiodotyrosin (DIT).

7. Two DIT molecules then undergo an oxidative condensation to form T4
Two DIT molecules then undergo an oxidative condensation to form T4.or one molecule of MIT couples with one molecule of DIT to form triiodothyronine
Each thyroglobulin molecule contains 30 thyroxin molecules & a few triiodothyronine molecules.in this form thyroid hormones are stored in the follicles for 2-3 months.
Release of thyroxin & triiodothyronine from the thyroid
gland :
The human thyroid secrets about 80µg (103 nmol)T4,& 4µg T3(7nmol).the thyroid cells ingest colloid by endocytosis .in the cells the globules of the colloid merge with lysosomes .the peptide bonds between the iodinated residuse & the thyroglobulin are broken by the proteases in the lysosomes &T4, T3,MIT,DIT are librated into the cytoplasm. the iodinated tyrosins are deiodinated by deiodinase enzyme ,T4&T3 pass into the circulation.

8. Figure 18.12 The Thyroid Follicles
Figure 18.12a

9. Physiologic functions of thyroid hormones
Thyroid hormones increase cellular metabolic activity the BMR increase to % above normal when large quantities of hormone are released .the rate of utilization of foods for energy is accelerated ,the rate of protein synthesis is increased . thyroid hormones increase the number& activity of the mitochondria .
thyroid hormones increase active transport of ions through cell membranes.
1.Effect of thyroid hormones on growth:
The effect of thyroid hormones on growth manifest in growing children.

10. 2.effects of thyroid hormones on specific bodily mechanisms:
In those who are hypothyroid the rate of growth is retarded.
In those who are hyperthyroid excessive skeletal growth occur causing the child to become taller at an earlier age.
An important effect is to promote growth & development of the brain during fetal life &for first few years of
postnatal life.
2.effects of thyroid hormones on specific bodily mechanisms:
Stimulation of carbohydrate metabolism , it causes rapid uptake of glucose by cells , increased glycolysis , enhanced gluconeogenesis , increased insulin secretion.
Stimulation of fat metabolism, decreases the concentrations of cholesterol,phospholipids, and triglycerides in the plasma
Increased requirement for vitamins.
Increased BMR.
Decreased body weight.

12. 3.Effect of thyroid hormones on cardiovascular system
Increased blood flow and cardiac output . Increased metabolism in the tissues causes more utilization of oxygen than normal & releases greater than normal metabolic end products from the tissues , these effects cause vasodilatation so it increase blood flow & as a consequence cardiac output is increased .
Increased heart rate thyroid hormone seems to have a direct effect on the excitability of the heart.
Increased heart strength ,The increased enzymatic activity caused by increased thyroid hormone production apparently increases the strength of the heart when only a slight excess of thyroid hormone is, but when increased markedly the strength of heart muscle decrease because of long-term excessive protein catabolism..
Normal arterial pressure : The mean arterial pressure usually remains about normal after administration of thyroid hormone. Because of increased blood flow
through the tissues between heartbeats, the pulse pressure is often increased, with the systolic pressure elevated in hyperthyroidism 10 to 15 mm Hg and the diastolic pressure reduced a corresponding amount.

13. 4. Increased respiration : the increase in utilization of oxygen & formation of co2 increases the rate & depth of respiration .
5. Increased G.I.T motility : thyroid hormone increases both the rate of secretion of digestive juices & the motility of G.I.T . Diarrhea often result from hyperthyroidism while constipation from hypothyroidism .
6. Excitatory effects on the C.N.S : the hyperthyroid subject have extreme nervousness , anxiety .
7. Effect on the function of the muscles : increased thyroid hormone weakens the muscles because of protein catabolism , while hypothyroidism causes the muscle to become relax slowly after contraction .
Muscle Tremor. One of the most characteristic signs of
hyperthyroidism is a fine muscle tremor.

14. 8. Effect on sleep : Because of the exhausting effect of thyroid hormone on the musculature and on the central nervous system, the hyperthyroid subject often has a feeling of constant tiredness, but because of the excitable effects of thyroid hormone on the synapses, it is difficult to sleep.. 9. Effect on other endocrine glands : increased thyroid hormones increase the rate of secretions of most endocrine glands . Increased thyroxin secretion increases the rate of glucose metabolism everywhere in the body and therefore causes a corresponding need for increased insulin secretion by the pancreas.

15. Effect on other endocrine glands
Also, thyroid hormone increases man metabolic activities related to bone formation and, aa consequence, increases the need for parathyroid hormone. Thyroid hormone also increases the rate at which adrenal glucocorticoids are inactivated by the liver. This leads to feedback increase in adrenocorticotropic hormone production by the anterior pituitary and, therefore, increased rate of glucocorticoid secretion by the adrenal glands.

16. Effect of Thyroid Hormone on Sexual Function
For normal sexual function, thyroid secretion needs to be approximately normal. In men, lack of thyroid hormone is likely to cause loss of libido; great excesses of the hormone, however, sometimes cause impotence. In women, lack of thyroid hormone often causes menorrhagia and polymenorrhea— that is, respectively, excessive and frequent menstrual bleeding. In the hyperthyroid woman, oligomenorrhea, which means greatly reduced bleeding, is common, and occasionally amenorrhea results.

17. Regulation of thyroid hormone secretion
The anterior pituitary secretion of TSH is controlled by TRH which secreted by nerve endings in the median emninence of the hypothalamus , then TRH is transported to the anterior pituitary by way of hypothalamic – hypophysial portal blood . TRH is a tripeptide amide , it stimulates the anterior pituitary to secrete TSH . TSH also called thyrotropin , is a glycoprotein with a molecular weight of about , this hormone increases the secretion of thyroxin & tri-iodothyronine by the thyroid gland . increased thyroid hormone in body fluids , decreases secretion of TSH by the anterior pituitary .

19. Diseases of the thyroid gland
Hyperthyroidism :
The thyroid gland is increased 2 – 3 times normal size and increase thyroid secretion .
Symptoms :
Excitability .
Intolerance to heat .
Increase sweating .
Weight loss .
Diarrhea .
Muscle weakness .
Nervousness .
Fatigue .
Tremor of hands .

20. Hypothyroidism :
Decreased thyroid hormone secretion .
symptoms :
Fatigue and sleep up to h / day .
Slow heart rate and decreased cardiac output , decreased blood volume .
Increased body weight .
Constipation .
Mental sluggishness .
Husky voice .
Decreased hair growth .

21. Thyroid Disorders
Figure 18.13

22. Grave’s disease Severe form of hyperthyroidism More common in women
Symptoms: strained and tense facial expression, exophthalmia, goiter, nervous irritability
goiter
exophthalmos

23. Face becomes swollen, weight increases and memory begins to fail
Cretinism
Myxedema
Develops early in infancy or childhood
Lack of mental/physical growth resulting in mental retardation and malformation
Sexual development and physical growth does not reach beyond 7-8 year old children
Normal development cannot be completely restored w/ tx.
Face becomes swollen, weight increases and memory begins to fail
Treatment is daily thyroid hormone
Follow-up tests to measure TSH blood levels are important