Thyroid gland (applied physiology)
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Thyroid Gland(Applied Physiology)DANISH HASSANLECTURER, UNIVERSITY OF SARGODHA
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Thyroid Gland DisordersHyper-Secretion1. Hyper-thyrodism
1. Graves Disease2. Exophthalmoses3. Goiter (Toxic
Goiter)
Hypo-Secretion1. Hypo-thyrodism
1. Myxedema2. Cretinism3. Goiter (Non-toxic
Goiter)
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1. Hyperthyroidism Increased secretion of thyroid hormones is called
hyperthyroidism.OR
Excessive functional activity of the thyroid gland, characterized by increased basal metabolism and disturbances in the activity of the autonomic nervous system as a result of excess thyroid hormone production.
The incidence is higher in women than in men.
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Causes of Hyperthyroidism Hyperthyroidism is caused by:
Graves’ disease Thyroid adenoma
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Graves’ Disease: An autoimmune disease characterized by secretion
of antibodies by the body to the receptor for thyroid-stimulating hormone (TSH), which results in chronic stimulation and production of T3 and T4 by the thyroid gland.
Accounts for 50–80% of cases of hyperthyroidism Normally, TSH combines with surface receptors of
thyroid cells and causes the synthesis and secretion of thyroid hormones.
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In Graves’ disease, the B lymphocytes (plasma cells) produce autoimmune antibodies called thyroid-stimulating autoantibodies (TSAbs).
These antibodies act like TSH by binding with membrane receptors of TSH and activating cAMP system of the thyroid follicular cells.
This results in hype-rsecretion of thyroid hormones.
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Antibodies act for a long time even up to 12 hours in contrast to that of TSH, which lasts only for an hour or so.
The high concentration of thyroid hormones caused by the antibodies suppresses the TSH production also.
So, the concentration of TSH is low or almost zero in plasma of most of the hyperthyroid patients
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Thyroid Adenoma Localized tumor develops in the thyroid tissue which
secretes large quantities of thyroid hormones. It is not associated with autoimmunity. As far as this adenoma remains active, the other
parts of thyroid gland cannot secrete the hormone. This is because, the hormone secreted from
adenoma depresses the production of TSH.
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Signs and Symptoms of Hyperthyroidism1. Intolerance to heat due to increased BMR2. Increased sweating due to vasodilatation3. Decreased body weight due to fat mobilization4. Diarrhea due to increased motility of GI tract5. Muscular weakness because of excess protein
catabolism
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7. Nervousness, extreme fatigue, inability to sleep, mild tremor in the hands and psychoneurotic symptoms such as hyperexcitability, extreme anxiety or worry.
8. Toxic goiter9. Oligomenorrhea or amenorrhea 10. Exophthalmos 11. Polycythemia
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12. Tachycardia and atrial fibrillation13. Systolic hypertension14. Cardiac failure.
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2. Exophthalmos Protrusion of eye balls is called exophthalmos. Also called proptosis (anterior displacement of
eye from the orbit) Most, but not all hyperthyroid patients develop
some degree of protrusion of eyeballs.
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Causes for Exophthalmos Occurs due to the edematous swelling of retro-
orbital tissues and degenerative changes in the extraocular muscles.
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Effect of Exophthalmos on Vision: Severe exophthalmic condition leads to blindness
because of two reasons:1. Protrusion of the eyeball, which stretches and
damages the optic nerve, resulting in blindness 2. Due to the protrusion of eyeballs, the eyelids cannot
be closed completely while blinking or during sleep. So, the constant exposure of eyeball to atmosphere causes dryness of the cornea, leading to irritation and infection.
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It finally results in ulceration of the cornea leading to blindness.
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1. Hypothyroidism Decreased secretion of thyroid hormones is
called hypothyroidism.
OR A condition resulting from insufficient thyroid
hormone action. It has an incidence of 2% in adult women and is
less common in men.
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Hypothyroidism leads to Myxedema (in adults) Cretinism (in children)
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Myxedema Myxedema is the hypothyroidism in adults. Characterized by generalized edematous
appearance, demonstrating bagginess under the eyes and swelling of the face
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Causes for Myxedema1. Genetic disorder 2. Iodine deficiency. 3. Deficiency of thyroid-stimulating hormone or
thyrotropin-releasing hormone.4. Autoimmune disease called Hashimoto’s thyroiditis,
which is common in late middle-aged women. 5. Inflammation of gland, called thyroiditis caused by
autoimmune antibodies.
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Signs and symptoms of Myxedema Typical feature of this disorder is an edematous
appearance throughout the body. It is associated with the following symptoms:
Swelling of the face Bagginess under the eyes
It does not make pits and the edema is hard. It is because of accumulation of proteins with
hyaluronic acid and chondroitin sulfate, which form a hard tissue with increased accumulation of fluid
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Atherosclerosis It is the hardening of the walls of arteries because of
accumulation of fat deposits and other substances. In myxedema, it occurs because of increased
plasma level of cholesterol which leads to deposition of cholesterol on the walls of the arteries.
Atherosclerosis produces arteriosclerosis, which refers to thickening and stiffening of arterial wall.
Arteriosclerosis causes hypertension.
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Other general features of hypothyroidism in adults are:1. Anemia2. Fatigue and muscular sluggishness3. Extreme somnolence with sleeping up to 14 to 16 hours
per day4. Menorrhagia and polymenorrhea5. Decreased cardiovascular functions such as reduction in
rate and force of contraction of the heart, cardiac output and blood volume
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6. Increase in body weight7. Depressed hair growth8. Scaliness of the skin9. Frog-like husky voice10. Cold intolerance11. Constipation12. Mental sluggishness
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Cretinism Cretinism is the hypothyroidism in children,
characterized by stunted growth. It results from
1. Congenital lack of a thyroid gland (congenital cretinism)
2. From failure of the thyroid gland to produce thyroid hormone because of a genetic defect of the gland
3. Iodine lack in the diet (Endemic cretinism).
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Causes for Cretinism Cretinism occurs due to congenital absence of
thyroid gland, genetic disorder or lack of iodine in the diet.
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Features of Cretinism A newborn baby with thyroid deficiency may appear
normal at the time of birth because thyroxine might have been supplied from mother.
But a few weeks after birth, the baby starts developing the signs like sluggish movements and croaking sound while crying.
Unless treated immediately, the baby will be mentally retarded permanently.
Skeletal growth is more affected than the soft tissues.
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So, there is stunted growth with bloated body. The tongue becomes so big that it hangs down with
dripping of saliva. The big tongue obstructs swallowing and breathing. The tongue produces characteristic guttural
breathing that may sometimes choke the baby. Disproportionate rate of growth, the soft tissues are
likely to enlarge excessively, giving the child with cretinism an obese, stocky, and short appearance.
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Cretin Vs DwarfCretinism
1. There is mental retardation
2. Different parts of the body are disproportionate.
3. Development of nervous system is abnormal
4. Reproductive function is affected in cretinism
Dwarfism1. No mental retardation
and 2. Different parts of body
are proportionate. Whereas, in dwarfism,
3. Development of nervous system is normal
4. Reproductive function may be normal in dwarfism.
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Goiter Enlargement of the thyroid gland. It occurs both in hypothyroidism and
hyperthyroidism.
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Goiter in Hyperthyroidism – Toxic Goiter Toxic goiter is the enlargement of thyroid gland
with increased secretion of thyroid hormones, caused by thyroid tumor.
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Goiter in Hypothyroidism – Non-toxic Goiter Enlargement of thyroid gland without increase in
hormone secretion. It is also called hypothyroid goiter. Based on the cause, the non-toxic hypothyroid
goiter is classified into two types.1. Endemic colloid goiter2. Idiopathic non-toxic goiter
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Endemic Colloid Goiter It is the non-toxic goiter caused by iodine deficiency. It is also called iodine deficiency goiter. Iodine deficiency occurs when intake is less than 50
μg/day. Because of lack of iodine, there is no formation of
hormones. By feedback mechanism, hypothalamus and anterior
pituitary are stimulated.
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It increases the secretion of TRH and TSH. TSH then causes the thyroid cells to secrete
tremendous amounts of thyroglobulin into the follicle
As there are no hormones to be cleaved, the thyroglobulin remains as it is and gets accumulated in the follicles of the gland.
This increases the size of gland
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Idiopathic Non-Toxic goiter Idiopathic non-toxic goiter is the goiter due to
unknown cause. Enlargement of thyroid gland occurs even without
iodine deficiency. The exact cause is not known. It is suggested that it may be due to thyroiditis and
deficiency of enzymes such as peroxidase, iodinase and deiodinase, which are required for thyroid hormone synthesis.
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Some foodstuffs contain goiterogenic substances (goitrogens) such as goitrin.
These substances contain antithyroid substances like propylthiouracil.
Goitrogens suppress the synthesis of thyroid hormones. Therefore, TSH secretion increases, resulting in
enlargement of the gland. Such goitrogens are found in vegetables like turnips
and cabbages. Soybean also contains some amount of goitrogens
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Thyroid Gland(Treatment of Thyroid Disorders)DANISH HASSANLECTURER, UNIVERSITY OF SARGODHA
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Treatment For Hyper-thyrodism
By using Antithyroid Substances Antithyroid substances are the drugs which
suppress the secretion of thyroid hormones. Hyperthyroidism in early stage can be treated by
anti-thyroid substances. Three well-known anti-thyroid substances are:
1. Thiocyanate2. Thiourylenes3. High concentration of inorganic iodides.
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Thiocyanate Prevents synthesis of thyroxine by inhibiting iodide
trapping. The active pump which transports iodide into the
thyroid cells, can transport thiocyanate ions also. So, administration of thiocyanate in high
concentrations causes competitive inhibition of iodide transport into the cell.
So, iodide trapping is inhibited, leading to the inhibition of synthesis of thyroxine.
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Thiourylenes Thiourylenes are the thiourea-related substances
such as propylthiouracil and methimazole, which prevent the formation of thyroid hormone from iodides and tyrosine.
It is achieved partly by blocking peroxidase enzyme activity and partly by blocking coupling of iodinated tyrosine to form either T3 or T4.
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The deficiency of the hormone increases the TSH secretion, which increases the size of thyroid gland with more secretion of thyroglobulin.
Thyroglobulin accumulates in the gland and causes enlargement of the gland, resulting in non-toxic goiter
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High Concentration of Inorganic Iodides Iodides in high concentration decrease all phases of
thyroid activity, including the release of hormones. So the size of the gland is also reduced with
decreased blood supply. Because of this, iodides are frequently administered
to hyperthyroid patients for 2 or 3 weeks, prior to surgical removal of the thyroid gland.
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By Surgical Removal In advanced cases of hyperthyroidism, treatment
by using antithyroid substances is not possible. So, thyroid gland of these patients must be
removed. Surgical removal of thyroid gland is called
thyroidectomy. Before surgery, the patient is prepared by
reducing the basal metabolic rate.
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It is done by injecting propylthiouracil for several weeks, until basal metabolic rate reaches almost the basal level.
The high concentration of iodides is administered for 2 weeks.
It decreases the size of the gland and blood supply to a very great extent. Because of these precautions, the mortality after the operation decreases very much
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Treatment For Hypothyroidism
The only treatment for hypothyroidism is the administration of thyroid extract or ingestion of pure thyroxine in the form of tablets, orally.
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Thyroid Function Tests Functional status of thyroid gland is assessed by
the following tests:1. Measurement of plasma level of T3 and T42. Measurement of TRH and TSH3. Measurement of basal metabolic rate