The thyroid gland is the largest endocrine gland of our body which regulates our overall growth and metabolism. It is endodermal in origin.
This gland is situated at the ventrolateral side of the joint of the trachea and larynx in the neck region of the man. It is bilobed or H-shaped or butterfly-shaped. Both of their lobes are connected by a non-glandular flap of a connective tissue called the isthmus.
Each lobe of the thyroid gland is made up of connective tissue. In it there are present so many follicles made up of glandular cells in connective tissue. These follicles are scattered in loose connective tissue, the stroma, and many round follicles of glandular cells.
A layer of cuboidal glandular cells is found in the wall of follicles. An iodized colloidal substance, Thyroglobulin is filled in the cavity of these follicles. Thyroglobulin is a glycoprotein in nature.
The thyroid is the only endocrine gland in the body that stores its hormone in its inactive state.
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Parafollicular cells or c – cells
Parafollicular cells are found in interstitial tissue in between the follicles and they secrete thyrocalcitonin (calcitonin) hormone which lacks iodine. It is proteinaceous in nature.
These cells are scattered between thyroid follicles in stromal tissue. These cells are of endocrine nature. These cells secrete thyrocalcitonin or calcitonin hormone (TCT) which lacks iodine. It is proteinaceous in nature.
Thyrocalcitonin enhances the deposition of calcium in bones and increases the rate of excretion of calcium in urine, thus reducing Calcium levels in the blood so also known as a hypocalcemic hormone.
This hormone is antagonistic to parathormone (PTH) or Collip’s hormone.
Production of hormones in follicles of the thyroid gland
- T3 = Tri-iodothyronine (20%)
- T4 = Tetra-iodothyronine (80%) or Thyroxine or calorigenic hormone.
T3 and T4 are derivatives of Tyrosine amino acids.
Secretion of T4 is comparatively more than T3 and T3 hormone is four times more effective than T4 hormone.
T4 converted into T3 on reaching the tissues.
Thyroid hormones in the form of thyroglobulin are stored in the follicles in an amount sufficient to supply the body with its normal requirements of thyroid hormone for 3 months.
The thyroid gland requires iodine “120 μg” every day for normal production of thyroxine.
Thyroxine is also known as a calorigenic hormone.
Functions of thyroxine
(A) Growth and Development
This hormone promotes physical (skeletal, muscular, and visceral), mental and sexual growth.
(B) Metabolism
(i) Fat: Enhances enzymatic activity for both syntheses & predominantly catabolism of fats (lipolysis).
(ii) Carbohydrate: Blood sugar increases, act as a diabetogenic hormone.
(iii) Protein: Both catabolism & anabolism but at an optimum concentration of thyroxin, anabolism is dominant.
(C) Basal metabolic rate (BMR)
(i) Thyroxine regulates the basal metabolic rate (BMR) in the body.
BMR: BMR refers to the minimum amount of energy in the form of calories that our body requires to complete its normal function.
BMR increases > body temperature increase > loses weight
The hormone enhances food oxidation rate, in the body cells as a result of that energy production is also increased in the form of calories so this hormone is also called calorigenic hormone.
(ii) Increase activity of Na+ – K+ ATPase. It increases the number of mitochondria in all the cells of the body.
It increases the consumption of oxygen by the cells of the body (Increase BMR).
(D) Blood: Stimulate erythropoiesis in the bone marrow.
(E) Water balance: Maintenance of water and electrolytes balance is also influenced by thyroid hormone.
(F) Central Nervous System (CNS): Helps in the development and maturation of the central nervous system.
(G) Gonads: Thyroxin also influences the menstrual cycle.
Thyroid disorders
Hypothyroidism
Simple goitre
Simple goitre is also known as colloidal goitre. If there is a deficiency of iodine in food then the thyroid gland tries to absorb more and more iodine from the blood and enlarges in size called simple goitre.
Goitre is found more abundantly in the persons who live on mountain slopes because iodine (present on the surface of that place) flows along with rainwater. When most of the people in slope areas show the symptoms of this disease, therefore, it is also called endemic goitre.
Persons who take seafood, never show the symptoms of goitre.
Thyroid dwarfism
Thyroid dwarfism is also known as cretinism. Hypothyroidism during pregnancy causes defective development and maturation of the growing baby leading to stunted growth, mental retardation (low intelligence quotient – I.Q. below 70 to 75, which is below average), abnormal skin, deaf-mutism, etc.
It affects physical, mental and sexual development.
Physical
a) Skeletal bones become dwarf.
b) Muscles becomes weak.
c) Visceral organ development may also get retarded.
Mental
It causes idiocy/imbecelity/mental retardation/low IQ.
Sexual
Causes infantile sex organs and no secondary sexual characters.
Thyroid myxoedema
Thyroid myxoedema is also known as Gull’s disease. In adults, hypothyroidism causes obesity, low HBR, lack of alertness, and the menstrual cycle becomes irregular in women.
Myxoedema – Painless swelling.
Hashimoto’s disease
It is a type of auto-immune disorder. In this disease, antibodies are formed against the thyroid gland and attack their own cells.
Symptoms
- Enlargement of the thyroid gland.
- Bulging or protrusion of eye ball occurs. This condition also called proptosis.
Gulls’s disease
Symptoms
- Loose and swollen skin.
- Falling of hairs occurs.
- Deposition of body occurs.
Hyperthyroidism
Due to cancer or the development of nodules in thyroid glands, the rate of synthesis and secretion of thyroid hormone is increased to abnormally high levels leading to a condition called hyperthyroidism which adversely affects the body physiology.
Exophthalmic goitre
Exophthalmic goitre is also known as Grave’s disease or Basedow’s disease or Thyrotoxicosis. An exophthalmic goitre is a form of hyperthyroidism, characterized by enlargement of the thyroid gland, protrusion of the eyeballs increased basal metabolic rate, and weight loss.
In this disease, a TSH-like antibody is formed which induces the thyroid to produce more and more thyroxine which gives negative feedback to the anterior pituitary to stop producing TSH. Hence, this process continues and results in exophthalmic goitre.
Plummer’s disease
It is a type of cancer in the thyroid gland. In this condition, nodule formation occurs and later it grows and divides its cells to form more nodules so, the thyroid becomes multi-nodulated.
Due to the formation of nodules, the thyroid becomes enlarged and produces an excess amount of thyroid hormones which results in hyperthyroidism and causes toxic adenoma or Plummer’s disease.
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