INTRODUCTION: Coordination of body functions are accomplished by chemical messengers. Hormones are the chemical messengers secreted by endocrine gland. The thyroid gland is one of the largest endocrine gland in the body and consists of two connected lobes. The thyroid is found in neck, below the thyroid cartilage. The thyroid gland controls how quickly the body uses energy, makes proteins, and controls how sensitive the body is to other hormones.MORPHOLOGY: The thyroid gland is a butterfly shaped organ and is composed of two cones like lobes or wings, lobus dexter (right lobe) and lobus sinister (left lobe), connected isthmus. Normally, it weighs about 20 to 40 gm in adult but, it is subjected to variations physiologically. The organ is situated on the anterior side of the neck, lying against and around the larynx and trachea, reaching the carotids oesophagus and carotid sheath.

HISTOLOGY OF THE THYROID GLAND: Thyroid gland is composed of large number of closed follicles. The follicles are lined with cuboidal epithelial cells. These epithelial cells are called the follicular cells. The follicular cavity is filled with a colloidal substance known as thyroglobulin which is secreted by the cuboidal epithelial cell. Actually, the synthesis of thyroid hormone takes place in the thyroglobulin. In between the follicles, the parfollicular cells are present HORMONES OF THE THYROID GLAND:A.The follicular cells synthesize two principle iodine containing hormones which are:1. Thyroxine which is 3. 5. 3'. 5' Tetra iodothyronine refferd to a T4. Daily secretion is about 80g. 2. Triiodothyronine or 3. 5. 3'. Triiodothyronine reffered to a T3. Daily secretion is about 4g

B.The parafollicular cells produce calcitonine(thyrocalcitonine) which influence calcium metabolism.It is a polypeptide hormone. The release of hormone is stimulated by high levels of ionized calcium in the blood.SYNTHESIS OF THYROID HORMONES: Iodine and tyrosine are essential for the formation of thyroid hormones. Iodine is consumed through diet. It is converted into iodine and absorbed from gastrointestinal tract. Tyrosine is also consumed through diet and, is absorbed from the gastrointestinal tract. For the synthesis of normal quantities of thyroid hormones, approximately an amount of 1mg of iodine is requried per week or 50 mg per year. To prevent iodine deficiency, common table salt is iodized with one part of sodium iodide to every 100,000 parts of sodium chloride.STAGES OF SYNTHESIS OF THYROID HORMONES:Various steps are involved in the formation of thyroid hormones. Thes are:Thyroglobulin secretionThyroglobulin is synthesized in endoplasmic reticulum and follows the secretory path way to enter the colloid of thyroid follicle by exocytosis. Iodide trapping or Iodide pumpThe basal membrane of the thyroid cells has a specific ability to transport the iodide from the blood into the interior of the cell. A sodium iodide Na/I symporter pumps iodide actively into the cells. Then this iodide enters the follicular lumen by the transporter pendrin. Thus, the process by which the iodide is transported is called iodide trapping or iodide pump. Oxidation of the iodideIodine has to be oxidized to elementary iodine. The oxidation of iodide into iodine occurs inside the cells. And the process of oxidation is promoted by the enzyme Peroxidases.Iodination of tyrosine Iodine has very reactive and iodinate the thyroglobulin at tyrosol residues in its protein chain. In conjugation, adjacent tyrosol residues are paired together. The entire complex re-enters the follicular cell by endocytosis. Proteolysis by various proteases liberates thyroxine and tri iodothyronine molecules which enters the blood.

Iodination of tyrosine occur in several stages. Tyrosine is iodized first into mono iodotyrosine. Two molecule of mono iodotyrosine combine to form di iodotyrosine. One mono iodotyrosine and one di iodotyrosine combine to form tri iodothyronine (T3). Two di iodotyrosine combine to form tetra iodothyronine (T4) which is thyroxine.

STORAGE OF THYROID HORMONE: After synthesis, the thyroid hormones remain in the form of vesicle along with thyroglobulin. Each thyroglobulin molecule contains 5 or 6 molecule of thyroxine. There is also an average 1 triiodothyronine molecule for every 10 molecule of thyroxine. In this way, i.e., in combination with thyroglobulin, the thyroid hormones can be stored for several months. And, thyroid gland is unique in this, as it is the only endocrine gland which can store its hormones for a long period.

TRANSPORT OF THYROID HORMONES IN THE BLOOD: The thyroid hormones are transported in blood in combination with the plasma proteins. The normal plasma level of total (tri-iodothyronine) T3 is 8g/dl and the total (tetra-iodothyronine) T4 is 0.15g/dl. There are three types of plasma protein for the transport in thyroid hormones in the blood. These are:1. Thyroxine binding globulin (TBG) This is a glycoprotein and in concentration in blood is 1 to 1.5 mg%. It has a great affinity of thyroxine and about one third of the hormone combines strongly with this protein.2. Thyroxine binding prealbumin This protein transports one fourth of the thyroid hormones.3. Albumin About one tenth of the thyroid hormones is transported by means of albumin.FUNCTION OF THYROID HORMONE: Effect on basal Metabolic rateThyroxine increases the metabolic activities of almost all tissues of the body except brain, retina, spleen and lungs. The basal metabolic rate (BMR) can increase to as much as 60 to 100% above the normal level when, large quantity of thyroxine secreted. And, the BMR can fall by 20 to 40% below the normal level when less quantity of thyroxine is secreted. This is because, thyroxine Increase the rate of food stuff in the peripheral cells. Effect on growth Thyroid hormones have general and specific effects on growth. Lack of thyroxine can arrest the growth and increase in thyroxine secretion can accelerate the growth of the body especially in growing children. Effect on body weightThyroxine is also essential for maintaining the weight of the body. Increase in thyroxine decreases the body weight and decrease in thyroxine increases the body weight. Effect on respiration Thyroxine increases the rate and force of respiration indirectly. The increased metabolic rate caused by thyroxine increases utilization of oxygen and formation of carbon dioxide. The two factors stimulate the respiratory centers to increase the rate and force of respiration. Effect on gastrointestinal tractThyroxine generally increases the appetite and food intake. It also increases the secretions and the movements of gastrointestinal tract. So, the lack of thyroxine causes constipation and increased secretion of thyroxine causes diarrhea. Effect on skeletal muscleThyroxine is essential for the normal activity of the skeletal muscles. Slight increase in thyroxine can make the muscle to work with more vigor. But, excessive secretion can cause weakness in the muscle due to catabolism of proteins. The muscles relax very slowly after the contraction. Hypothyroidism can also cause fine muscular tremor. The tremor can occur at the frequency of 10 to 15 times per second. This is due to thyroxine induced excessive neuronal activity, which controls the muscle. And, the lack of thyroxine can make muscles more sluggish. Effect on sleepHyper secretion of thyroxine causes excessive stimulation of the muscles and central nervous system. So, the person feels tired and exhausted and feels like sleeping. But, the person cannot sleep because of the stimulatory effect of thyroxine neurons. On the other hand, hyposecretion of thyroxine causes excessive sleep. The condition with excessive sleep is called the somnolence

LactationIn hypothyroidism, milk secretion is decreased. Administrations of thyroid hormones increase lactation. Effect on other endocrine glandBecause of its effects on metabolism, thyroxine increases the demand for secretions of other endocrine gland.REGULATION OF SECRETION OF THYROID GLAND The secretion of thyroid hormone is controlled through the feedback mechanism. Thyrotropic releasing hormone (TRH) is secreted from hypothalamus. As a result, anterior pituitary is stimulated and Thyroid stimulating hormone (TSH) is produced. TSH stimulates the thyroid gland to synthesize and release thyroxine. When the blood level of thyroid hormone increases, the release of TRH is suppressed, leading to stoppage of the release of thyroid hormones.

EFFECT ON THYROID STIMULATING HORMONE (TSH ) ON THYROID GLAND: Thyroid stimulating hormone is necessary for the growth and the secretory activity of the thyroid gland. Its actions on thyroid gland are as follows: It increases the number of thyroid cells, which are cuboidal in nature, and then it converts these cuboidal cells into columnar cells and causes the development of thyroid follicles. It increases the size and secretory activity of the cells. It increases the iodide pump and iodide trapping in the cells. It increases the thyroglobulin secretion into the follicles. It increases proteolysis of thyroglobulin, by which, release of hormone is enhanced and the colloidal substance is decreased.DISORDER OF THYROID GLAND:1. HYPERTHYROIDISM It is also called as over activity of thyroid gland. It is a condition in which the thyroid gland produces and secretes excessive amount of thyroxine and triiodothyronin. Causes of hyperthyroidism Hyperthyroidism occurs due to presence of TSH like substances in the blood. This is proved by radio immunoassay studies. The concentration of TSH was found to be less or almost zero in the plasma of the most of the hypothyroid patients. These TSH like substances are immunoglobulin antibodies, which bind with same membrane receptors of TSH. These activate the cyclic AMP system of the cells and cause in hyperthyroidism. Symptoms of hyperthyroidism Increase in basal metabolic rate(BMR) and appetite Excessive sweating occurs due to vasodilatation Muscular weakness Eye balls are protruded Mental growth is very sharp and patient feels restless Emotional and nervous excitability There is enlargement of the thyroid gland, which is called the toxic goiter2. Hypothyroidism Reduced activity of thyroid gland causes hypothyroidism. Hypothyroidism leads to myxedema in adults and cretinism in children. Myxedema It is a condition that appears in the adult life due to deficiency in the secretions of thyroxine. Symptoms Skin becomes swollen, puffy, and dry Bagginess under the eye Hairs become scarce, brittle and coarse Low BMR, low blood sugar and iodine Appetite is reduced Body temperature becomes lower and body weight increases.

Cretinism Hypothyroidism in children leads to a condition known as cretinism. Cretinism may occur due to congenital lack of thyroid gland, genetic disorder or lack of iodide in the diet

Symptoms Stunted growth Low intelligence Slow heart rate Slow bone growth The patient is mentally retarded and a sterile one.

CONCLUSION

Thyroid is the only gland of the body which has has the capacity to store and utilize iodine. It is very essential for growth and development of the body. So, it is a important endocrine gland, and we should take proper nutrient by which the gland remain well.