Thyroid is an endocrine gland situated at the root of the neck on either side of the trachea. It has two lobes, which are connected in the middle by an isthmus .
Thyroid gland is composed of large number of closed follicles. These follicles are lined with cuboidal epithelial cells, which are called the follicular cells. Follicular cavity is filled with a colloidal substance known as thyroglobulin, which is secreted by the follicular cells Follicular cells also secrete tetraiodothyronine (T, or thyroxine) and tri-iodothyronine (T). In between the follicles, the parafollicular cells are present. These cells secrete calcitonin.
It weighs about 20 to 40 g in adults. Thyroid is larger in females than in males. The structure and the function of the thyroid gland change in different stages of the sexual cycle in females.
Its function increases slightly during pregnancy and lactation and decreases during menopause.
HORMONES OF THYROID GLAND
Thyroid gland secretes three hormons
1. Tetraodothyronine or T4(thyroxin
2. Tri-iodothyronine or T3
FUNCTIONS OF THYROID HORMONES
Thyroid hormones have two major effects on the body: 1. To increase basal metabolic rate.
II. To stimulate growth in children.
The actions of thyroid hormones are:
1-BASAL METABOLIC RATE (BMR)
Thyroxine increases the metabolic activities in most of the body tissues, except brain, retina, spleen, testes and lungs. It increases BMR by increasing the oxygen consumption of the tissues. The action that increases the BMR is called calorigenic action.
In hyperthyroidism, BMR increases by about 60% to 100% above the normal level and in hypothyroidism it falls by 20% to 40% below the normal level.
2- FAT METABOLISM
Thyroxine decreases the fat storage by mobilizing it from adipose tissues and fat depots. The mobilized fat is converted into free fatty acid and transported by blood. Thus, thyroxine increases the free fatty acid level in blood.
Thyroxine increases the formation of many enzymes. Since vitamins form essential parts of the enzymes, it is believed that the vitamins may be utilized during the formation of the enzymes. Hence, vitamin deficiency is possible during hypersecretion of thyroxine.
4. BODY TEMPERATURE
Thyroid hormone increases the heat production in the body, by accelerating various cellular metabolic processesand increasing BMR. It is called thyroid hormone induced thermogenesis. During hypersecretion of thyroxine, the body temperature increases greatly, resulting in excess sweating.
Thyroid hormones have general and specific effects on growth. Increase in thyroxine secretion accelerates the growth of the body, especially in growing children. Lack of thyroxine arrests the growth. At the same time, thyroxine causes early closure of epiphysis. So, the height of the individual may be slightly less in hypothyroidism.
Thyroxine is more important to promote growth and development of brain during fetal life and first few years of postnatal life. Deficiency of thyroid hormones during this period leads to mental retardation.
6. BODY WEIGHT
Thyroxine is essential for maintaining the body weight Increase in thyroxine secretion decreases the body weight and fat storage. Decrease in thyroxine secretion increases the body weight because of fat deposition.
Thyroxine accelerates erythropoietic activity and increases blood volume. It is one of the important general factors necessary for erythropoiesis. Polycythemia is common in hyperthyroidism.
10. CARDIOVASCULAR SYSTEM
Thyroxine increases the overall activity of cardiovascular system.
i. On Heart Rate
Thyroxine acts directly on heart and increases the heart rate. It is an important clinical investigation for diagnosis of hypothyroidism and hyperthyroidism.
ii. On the Force of Contraction of the Heart
Due to its effect on enzymatic activity, thyroxine generally increases the force of contraction of the heart. But in hyperthyroidism or in thyrotoxicosis, the heart may become weak due to excess activity and protein catabolism. So, the patient may die of cardiac decompensation.
Cardiac decompensation refers to failure of the heart to maintain adequate circulation associated with dyspnea, venous engorgement (veins overfilled with blood) and edema.
11-CENTRAL NERVOUS SYSTEM
Thyroxine is very essential for the development maintenance of normal functioning of central nervous system (CNS).
i. On Development of Central Nervous System
Thyroxine is very important to promote growth and development of the brain during fetal life and during the first few years of postnatal life. Thyroid deficiency in infants results in abnormal development of synapses, defective myelination and mental retardation.
ii. On the Normal Function of Central
Thyroxine is a stimulating factor for the central nervous system, particularly the brain. So, the normal functioning of the brain needs the presence of thyroxine. Thyroxine also increases the blood flow to brain.
Normal thyroxine level is necessary to maintain normal sleep pattern. Hypersecretion of thyroxine causes excessive stimulation of the muscles and central nervous system. So, the person feels tired, exhausted and feels like sleeping. But, the person cannot sleep because of the stimulatory effect of thyroxine on neurons. On the other hand, hyposecretion of thyroxine causes somnolence.
13- SEXUAL FUNCTION
Normal thyroxine level is essential for normal sexual function. In men, hypothyroidism leads to complete loss of libido (sexual drive) and hyperthyroidism leads to impotence.
In women, hypothyroidism causes menorrhagia and polymenorrhea (Chapter 80). In some women, it causes irregular menstruation and occasionally amenorrhea. Hyperthyroidism in women leads to oligomenorrhea and sometimes amenorrhea.
14- OTHER ENDOCRINE GLANDS
Because of its metabolic effects, thyroxine increases the demand for secretion by other endocrine glands.
Increased secretion of thyroid hormones is called hyperthyroidism.
Causes of Hyperthyroidism Hyperthyroidism is caused by:
1. Graves’ disease
2. Thyroid adenoma.
1. Graves’ disease
Graves’ disease is an autoimmune disease and it is the most common cause of hyperthyroidism. Normally. TSH combines with surface receptors of thyroid cells and causes the synthesis and secretion of thyroid hormones. 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 hypersecretion of thyroid hormones.
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.
2. Thyroid adenoma
Sometimes, a localized tumor develops in the thyroid tissue. It is known as thyroid adenoma and it 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.
Signs and Symptoms of Hyperthyroidism
1. Intolerance to heat as the body produces lot of heat due to increased basal metabolic rate caused by excess of thyroxine.
2. Increased sweating due to vasodilatation
3. Decreased body weight due to fat mobilization
4. Diarrhea due to increased motility of GI tract
5. Muscular weakness because of excess protein catabolism
6. Nervousness, extreme fatigue, inability to sleep, mild tremor in the hands and psychoneurotic symptoms. such as hyperexcitability, extreme anxiety or worry. All these symptoms are due to the excess stimulation of neurons in the central nervous system
7. Toxic goiter
8. Oligomenorrhea or amenorrhea
9. Exophthalmos (see below)
11. Tachycardia and atrial fibrillation
12 Systolic hyperthyroidism
13 Cardiac failure
TREATMENT FOR HYPERTHYROIDISM
1. By using Antithyroid Substances
Antithyroid substances are the drugs which suppress the secretion of thyroid hormones. Hyperthyroidism in early stage can be treated by antithyroid substances. Three well-known antithyroid substances are:
iii. High concentration of inorganic iodides.
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.
Thioureylenes 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 T, or T..
During the use of these two antithyroid substances, even though the synthesis of thyroid hormone is inhibited, the formation of thyroglobulin is not stopped. 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.
iii. High concentration of inorganic iodides
lodides 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.
2. 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.
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.
THYROID FUNCTION TESTS
Functional status of thyroid gland is assessed by the following tests:
1. Measurement of plasma level of T, and T.: For hyperthyroidism or hypothyroidism, the most accurate diagnostic test is the direct measurement of concentration of “free” thyroid hormones in the plasma, i.e. T, and T
2. Measurement of TRH and TSH: There is almost total absence of these two hormones in hyperthyroidism. It is because of negative feedback mechanism, by the increased level of thyroid hormones.
3. Measurement of basal metabolic rate: In hyper thyroidism, basal metabolic rate is increased by about 30% to 60%. Basal metabolic rate is decreased in hypothyroidism by 20% to 40%.
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