Thyroid Gland: Hormones, Functions and Disorders

The thyroid gland is a butterfly-shaped endocrine gland. It is brownish-red that rich in blood vessels. Vital nerves that are responsible for voice quality also pass through the thyroid gland.

The thyroid gland is found in all vertebrate, but they are quite variable to shape and anatomical position. In some lower vertebrates, only thyroid follicles are present.

In human, the thyroid gland consists of two lobes that lie on either side of the roof of the trachea and are usually connected by a thin isthmus extending over the anterior surface of the second, third and fourth tracheal rings.

Two layers of fibrous connective tissues cover the thyroid. The gland is about two inches (5 cm) long. The weight of the normal thyroid gland of the adult people ranges from 20-60 gm. The gland is actually situated in front of the throat just below the thyroid cartilage, known as the Adam`s Apple.

The thyroid consists of two fairly symmetrical lateral lobes. Each lobe measures about 5 x 2 x 2 cm in size. The two lobes are connected by a strip of tissue, known as the isthmus. Each lobe consists of many follicles of variable size. Each follicle is lined by a single layer of granular cubical epithelium and filled with protein material, called colloid. Thyroid follicles contain a lesser number of high cubical mitochondria rich-cells. These are called parafollicular cells.

Thyroid gland
Thyroid gland

You might also read: Exocrine glands Vs Endocrine glands

Hormones of Thyroid Gland

The thyroid gland contains two types of cells found in the thyroid follicles and secretes the following three types of hormones:

Thyroxine: It is also known as T4, which is secreted by principal cubical epithelial cells.

Tri-iodothyronine: It is also known as T3, which is also produced by principal cubical epithelial cells.

Thyrocalcitonin: It is produced by the parafollicular cells.

Thyroxine and Tri-iodothyronine

These two hormones are iodinated derivatives of tyrosin (amino acid). They are generally referred to as thyroid hormones. Thyroid gland performs the following functions through these two hormones. It has been observed that tri-iodothyronine is more active than thyroxin.

Functions of Thyroid Hormones

  • Thyroid hormones are known as calorigenic hormones because they increase oxygen uptake and metabolism by the tissues and thus accelerate energy production.
  • They increase the sugar level in the blood. In a low dose, it causes protein synthesis, but in a high dose, it depresses protein synthesis.
  • Thyroxine decreases serum cholesterol and phospholipids.
  • Thyroxine helps in brain development, bone health and muscle control.
  • Thyroid hormones help in the skeletal, muscular, sexual and mental growth.
  • Thyroxine increases heart rate, cardiac output and blood pressure. It also dilates peripheral vessels.
  • Thyroid hormones increase the utilization of O2 and the formation of CO2. These effects increase the rate and depth of respiration.
  • Thyroxine increases absorption of foods, secretion of digestive juices and movement of the gastrointestinal tract.
  • Thyroid hormones help in the development of RBC. Hypofunctions of the thyroid cause anaemia.
  • By the calorigenic effect of thyroxine, body temperature increases, and thus it regulates body temperature.
  • It regulates central and peripheral nervous systems.
  • It controls the menstrual cycle in the female.
  • It also helps in metamorphosis. For example, the tadpole larva develops into a toad by the activities of the thyroid gland.

Hypofunction of Thyroid Gland or Hypothyroidism

Less secretion of thyroid gland produces cretinism in infancy or childhood and myxoedema in adults.


It is the condition in children due to severe thyroid deficiency. It is also known as congenital iodine deficiency syndrome. It hampers both mental and physical development, if untreated. It mainly occurs due to dietary iodine deficiency. Many people suffer from this congenital iodine deficiency syndrome around the world. It is a significant public health problem in many countries of the world. Many developed countries already have eliminated through iodine supplementation of food.

The most characteristic features of this defect are:

  • Stunted growth,
  • Goitre,
  • Inadequate length in infants,
  • Deformed bone and teeth,
  • Reduced adult stature,
  • Hair loss,
  • Rough, dry, wrinkled skin,
  • Bloated idiotic-look face,
  • Protruding large tongue and umbilicus,
  • Pot-bellied abdomen,
  • Impeded ovulation,
  • Sex organs and sex glands degenerate,
  • Mental deterioration,
  • Sex characters are retarded,
  • Neurological impairment,


This disease generally occurs more in adult female than male due to decrease or absence of thyroid hormone. It is also known as severely advanced hypothyroidism. It happens when the body does not make sufficient thyroid hormone. If untreated or undiagnosed, it causes severe advanced hypothyroidism. Critically advanced hypothyroidism can lead to Myxedema crisis, a life-threatening situation.

Causes of Myxedema (hypothyroidism)

  • Removal of the thyroid gland due to surgical operation,
  • Medication due to cancer treatments,
  • Radiation therapy can cause decreasing hormone production,
  • Iodine deficiency or an excess of iodine,
  • Hashimoto’s disease,
  • Sudden illness or infection,
  • Pregnancy, etc

Symptoms of Myxedema

  • The face appears as swollen puffy oedematous and thus exhibits Mongoloid appearance,
  • The thick dry skin, hairs falling from the axilla, pubis and head,
  • Swelling of the tongue and larynx, etc.,
  • Degeneration of sex dullness,
  • Loss of memory,
  • Slowing of heart rate,
  • Low blood pressure, etc.,
  • Sudden illness, like a heart attack or stroke,
  • Trauma
  • Stress,
  • Exposure to cold,
  • Low blood oxygen levels,
  • Decreasing breathing,
  • Decreasing blood sodium levels,
  • Shock,
  • Increasing CO2 levels in the blood,
  • Decreasing body temperature,
  • Confusion or mental slowness,
  • Constipation,
  • Weight gain,
  • Drooping eye,
  • Coma,

Hyperfunctions of Thyroid or Hyperthyroidism

This clinical syndrome due to hypersecretion of the thyroid gland is known as exophthalmic goitre or Graves disease.


  • The enlargement of the thyroid gland (swelling of neck),
  • Protrusion of eyeball occurs with less twinkling of eyelids. It is due to deposition of fat in the retro-ocular region.
  • The decrease in body weight,
  • Soft moist and flushed skin,
  • Increasing heart rate, cardiac output,
  • Rising of blood sugar,
  • A sharp mental condition, etc.


It is also known as calcitonin. It is a protein hormone secreted from the parafollicular cells or C-cells (mitochondria-rich cells) of the thyroid gland. It is also secreted from the cells of the glandular ultimobranchial bodies in fishes, birds, and other non-mammalian vertebrates.

Functions of Thyrocalcitonin

  • It helps to rise of calcium level in the blood.
  • It lowers the level of blood calcium and phosphate.
  • It promotes the formation of bone.

Concluding Remarks

The thyroid gland is a very important endocrine gland that is situated in your neck region. It produces three hormones such as thyroxine (T4), tri-iodothyronine (T3) and thyrocalcitonin. Among them, T3 and T4 are the most important hormones which play an important role to do normal function in your body.

There are many thyroid disorders due to abnormal secretion from the thyroid gland. These disorders occur in babies, children, teenagers and adults. It is estimated that about one in 20 people suffer from some kind of thyroid disorder. If untreated or undiagnosed, it can lead to a severe, life-threatening situation.