Thyroid Gland: Origin, Location and Structure

In this article we will discuss about:- 1. Origin of Thyroid Gland 2. Location and Structure of Thyroid Gland 3. Hormones.

Origin of Thyroid Gland:

It develops from the endoderm of the embryo.

Location and Structure of Thyroid Gland:

The thyroid gland is the largest endocrine gland located anterior to the thyroid cartilage of the larynx in the neck. The gland is well supplied with blood vessels. It is bilobed organ. The two lobes are connected by a narrow structure called the isthmus. The microscopic structure of the thyroid gland shows thyroid follicles com­posed of cubical epithelium and filled with a homogenous material called colloid.

Small amount of loose connective tissue forms stroma of the gland. Besides containing blood capillaries, the stroma contains small clusters of specialized Para follicular cells or ‘C’ cells. The thyroid gland is the only gland that stores hormones in large quantities for about two months.

Hormones of Thyroid Gland:

The thyroid gland secretes three hormones. Thyroxine (tetraiodothyronine or T₄), and tri­iodothyronine or T₃ are secreted by the thyroid follicular cells. Thyrocalcitonin is secreted by the С-cells of the thyroid gland. This gland is stimulated to secrete its hor­mones by thyroid stimulating hormone (also called thyrotropin) secreted by the anterior lobe of pituitary gland.

(i) Thyroxine (T₄) and Tri­iodothyronine (T₃):

T₄ and T₃ contain four and three atoms of iodine respectively, there­fore, they are named so. T₃ is secreted in smaller amounts but it is more active and several times more potent than T₄. T₄ is converted to T₃ by removal of one iodine in the liver, kidneys and some other tissues. Since both T₄ and T₃ have similar effects on the target cells, they are generally con­sidered together under the name, thyroid hormone (TH).

The thyroid gland is the only gland that stores its hormones in large quantity. T₄ and T₃ are synthesised by attaching iodine to tyrosine amino acid.

The functions of thyroxine (T₄) and tri-iodothyronine (T₃) are as follows.

(a) They regulate the metabolic rate of the body and thus maintain basal metabolic rate (BMR).

(b) They stimulate protein synthesis and, therefore, promote growth of the body tissues.

(c) They regulate the development of mental faculties.

(d) As they increase heat production, thus they maintain body temperature.

(e) They help in metamorphosis of tadpole into adult frog. If thyroid gland of the tadpole (larva) is removed, the larva fails to change into an adult.

(f) They increase action of neurotransmitters like adrenaline and noradrenaline.

(ii) Thyrocalcitonin (TCT):

It is secreted when calcium level is high in the blood. It then lowers the calcium level by suppressing release of calcium ions from the bones. Thus calciton has an action opposite to that of the parathyroid hormone on calcium metabolism. Calcitonin is a peptide which contains 32 amino acids.

Thyroid Disorders:

(A) Hyperthyroidism (Hyper secretion of thyroid hormone).

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 eye balls because of fluid accumu­lation behind them), loss of weight, slightly rise in the body temperature, excitability, rapid heartbeat, nervousness and restlessness.

(B) Hypothyroidism (Hypo secretion of thyroid hormone):

(a) Cretinism:

This disorder is caused by deficiency of thyroid hormone in infants. A cretin has slow body growth and mental development of reduced metabolic rate.

Other symp­toms of this disorder are slow heart beat, lower blood pressure, decrease in temperature, stunted growth, pot-belly, pigeon chest and protruding tongue and retarded sexual development. This disease can be treated by an early administration of thyroid hormones.

(b) Myxoedema or Gull’s disease:

It is caused by deficiency of thyroid hormone in adults. This disease is characterized by puffy appearance due to accumulation of fat in the subcutaneous tissue because of low metabolic rate. The patient lacks alertness, intelligence and initiative. He also suffers from slow heart beat, low body temperature and regarded sexual development. This disease can be treated by administration of thyroid hormones.

(c) Simple Goitre:

It is caused by deficiency of iodine in 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. Addition of iodine to the table salt prevents this disease.

(d) 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.