The thyroid gland is a butterfly-shaped organ and is composed of two cone-like lobes called the right lobe and left lobe, connected via the isthmus.
It develops from the endoderm.
It is located anterior to the thyroid cartilage of the larynx of the neck.
It is composed of thyroid follicles made up of cuboidal epithelium and filled with homogeneous material called colloid.
The stroma of the thyroid gland is made up of loose connective tissue.
The stroma contains blood capillaries and parafollicular cells or C cells.
It is the only gland that stores hormones in large quantities.

Hormones of the Thyroid Gland:

Thyroxine (T4) and Triiodothyronine (T3):

Thyroxine (T4) and Triiodothyronine (T3):
T4 contains four atoms of iodine and T3 contains three atoms of iodine.
T3 is active and potent form and is formed by the removal of one iodine from T4 in liver, kidneys and other tissues.
These are synthesised from tyrosine amino acids
The functions are as follows:
They regulate basal metabolic rate of the body and thus maintain basal metabolic rate (BMR)
They stimulate protein synthesis and thus promote growth of the body tissues
They maintain body temperature
They help in metamorphosis of tadpole into adult frog
They increase action of neurotransmitters like adrenaline and noradrenaline

Thyrocalcitonin:

It is secreted when calcium levels are high in the blood.
It lowers the levels of calcium by suppressing the release of calcium ions from the bones.
It is a peptide hormone.

Thyroid Disorders

Hyperthyroidism:

Hypersecretion of thyroid hormone is also called Exophthalmic goitre or Graves’ disease or Basedow’s disease or Parry’s disease.
It is a thyroid enlargement (goitre) in which the thyroid secretes excessive amount of thyroid hormone.
It is characterised by exophthalmia (protrusion of eyeballs because of fluid accumulation behind them), loss of weight, a slight rise in the body temperature, excitability, rapid heartbeat, nervousness and restlessness.

Hypothyroidism:

Cretinism:
- This disorder is caused by a deficiency of thyroid hormone in infants.
- Affected infants have slow body growth and mental development, and reduced metabolic rate
Myxoedema or Gull’s disease:
- It is caused by a deficiency of thyroid hormone in adults.
- This disease is characterized by puffy appearance due to accumulation of fat in the subcutaneous tissue because of a low metabolic rate.
- The patient lacks alertness, intelligence and initiative.
- He also suffers from slow heartbeat, low body temperature and retarded sexual development.
Simple Goitre:
- It is caused by a deficiency of iodine in the diet because iodine is needed for the synthesis of thyroid hormone.
- It causes thyroid enlargement.
- It may lead to cretinism or myxoedema.
- This disease is common in hilly areas.
Hashimoto’s disease:
- In this disease all the aspects of thyroid function are impaired.
- It is an autoimmune disease in which the thyroid gland is destroyed by autoimmunity.