Information about Thyroid endocrine gland and its functions

Do you know which is the largest endocrine gland of human body? In this article you will find a detail explanation of thyroid gland. You will find structure of thyroid gland. you will also find different hormones secreted by thyroid gland and diseases due to hypo and hyper secretion of thyroid gland.


Thyroid gland develops from the endoderm of the embryo. The thyroid gland is the largest endocrine gland of the human body which is located on the ventral side of the neck just below the larynx. Thyroid gland surrounds the front of the larynx and upper part of the trachea in the neck. Thyroid has two lobes on either side of the trachea.

Structure of thyroid gland

The thyroid gland is a pink coloured, H-shaped, bi-lobed and largest gland of the human body. The lobes of the thyroid gland are connected by an isthmus. In normal healthy person, the weight of thyroid gland is about 25 gm. The thyroid gland is larger in female than in male. The thyroid is profusely supplied with blood vessels. The gland consists of a large number of small closed follicles or acini of about 200 mili-micron in diameter. Each follicle is lined by a secretory epithelium of cuboidal or low columnar type. These follicles are held together by areolar tissue. Each follicle contains a clear viscid proteinaceous amber coloured colloid which normally comprises the greater part of the thyroid mass. The colloid consists mainly of iodinated thyroglobulin. In poly breeders like human beings the gland functions almost uniformly but in seasonal breeders, the gland is active at one stage and inactive at the other.

Hormones of thyroid gland

Thyroid gland secretes following hormones:

  1. Thyroxine or tetra-iodothyronine (T4)

  2. Tri-iodothyronine (T3)

  3. Thyrocalcitonin (TCT)

Thyroxine and tri-iodothyronine

Both of these hormones are iodine containing forms of amino acid tyrosine. They are stored in the colloids that fill the follicle and are released into the blood when required. Chemically thyroxine is tetra iodothyronine and also found as tri-iodothyronine. T 3 is more active (3 to 4 times) than T4. Both are iodine containing amine hormones. T 3 has 10% iodine while T 4 has 65% iodine. Thyroxine hormone was isolated by Kochar (1916) and it is crystalized by Kendall (1919). On reaching tissue 50% of T4 get converted into T3. Both of these hormones are secreted by follicular cells of thyroid gland.

Functions of thyroxin hormone

The functions of thyroxine hormone are as follows :

  • Thyroxine hormone regulates the rate of metabolic activities and growth.

  • The main effect of thyroxine is to speed up the oxidative energy releasing processes.

  • Thyroxine hormone plays an important role in the metamorphosis of amphibians and other vertebrates.

  • In the absence of thyroxine, metamorphosis does not occur and the animal remains int he larval stage.

  • Thyroxine hormone regulates the body temperature.

Thyrocalcitonin (TCT)

Thyrocalcitonin (TCT) hormone is secreted by the parafollicular cells (='C' cells) present in the stroma of the thyroid gland. Thyrocalcitonin is a protein composed of thirty-two amino acids. Thyrocalcitonin is a hypocalcemic, hypophosphatemic hormone. Thyrocalcitonin was discovered by H. H. Corp in 1962.

Functions of thyrocalcitonin

The functions of thyrocalcitonin hormones are:

  • Thyrocalcitonin hormone lowers the blood calcium controlling hypercalcemia but increases the calcification of bones.

  • Thyrocalcitonin acts mainly on bones causing inhibition of bones resorption in young ones.

  • Thyrocalcitonin controls the growth and differentiation.

Hyposecretion of thyroid

The hyposecretion of thyroid causing the following diseases:

  1. Cretinism: In the young mammals' deficiency of the thyroid hormone leads to cretinism, a disease characterized by dwarfness, deformed teeth and bones, pot belly, protruding tongue and dribbling saliva with an idiotic look and retarded mental growth. A Swiss scientist cum surgeon, Kocher (1883) unknowingly removed the thyroid gland of 16 patients by surgery. After a few days, all the patients developed the symptoms of cretinism. Later on, Moore (1891) observed that if the patients suffering from cretinism are orally provided with the extract of the thyroid gland or if they eat the thyroid gland of other animals they become healthy.

  2. Myxoedema: Deficiency of thyroid in adult human beings produces Myxoedema or Gull's disease characterized by swollen face, mental dullness, lowered B.M.R., dry skin, loss of appetite and impotency.

  3. Simple goitre: Whenever the amount of iodine is decreased in the body, the gland is enlarged in the size. The number of follicles is increased but the total production of hormone remains normal, so there is a marked enlargement of the gland which forms simple goitre or endemic goitre or colloid goitre.

  4. Hashimoto's disease: In this disease, all the aspects of thyroid functions are impaired. It is an autoimmune disease in which the thyroid gland is destroyed by autoimmunity.

Hypersecretion of thyroid

The hypersecretion of thyroid gland causing the following diseases:

  1. Exophthalmic goitre or Grave's disease: The enlargement may also be due to over stimulation of the gland by TSH. The patient of simple goitre is functionally normal but structurally a large gland. At this stage, if the supply of iodine becomes normal it results into exophthalmic goitre or Grave's disease characterized by the protrusion of the eyeball. This is due to the excess of thyroid hormone or hyperthyroidism which has symptoms like enlarged thyroid gland, increased B.M.R. and body temperature, protruded eyeball, etc. To prevent goitre daily dietary iodine intake should be about 1.1 mg.

  2. Due to increase in metabolic activities body temperature rises.

  3. Small tumour like structures are formed in the thyroid gland. This disease is known as Plummer's disease.

  4. Due to increase in appetite, intake of food is increased which results in an increase in weight.

  5. Metabolism rate is increased manyfold. As a result, the person becomes more sensitive and short tempered.

  6. Rate of heart beat is increased.

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Author: Natarajan15 Nov 2017 Member Level: Diamond   Points : 5

A nice summary of thyroid gland and its role in humans. The thyroid hormones are important for day to day metabolic functions of the body and hence often during times of increased demand ( puberty,pregnancy) the gland enlarges and hypo hypothyroidism can develop. Since iodine is crucial to the production of the thyroid hormones, widespread use of iodized salt has led to a decrease in thyroid disease(goiters). In current obstetric practice,it has become a must to monitor the thyroid hormone levels to ensure the health of the mother and the growing baby inside her.
Many of the diseases of thyroid (hypo function and hyper function) can be managed with tablets. Surgery is needed for complex cases of hyper function, for large goiters and when there is a diagnosis of cancer. When in doubt, see your local family physician or an endocrinologist.

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