Question

Name the $\mathop T\nolimits_3 $ and $\mathop T\nolimits_4 $ components of the, thyroid hormone. Explain their specific action.

Answer 1

Hint: The thyroid gland is the biggest and largest endocrine gland. The gland is butterfly-shaped or H-shaped. It is made up of two lateral lobes which are present on either side of the trachea. These lobes remain connected with each other with a thin flap of connective tissue called the isthmus.

Complete answer:
The thyroid gland or Iodine gland is made up of follicles and stromal tissues. These follicles are held with each other with the help of areolar connective tissue. Each thyroid follicle is composed of follicular cells, enclosing a cavity.
 The follicular cells present in the follicles synthesize two hormones, namely, tetraiodothyronine or thyroxine ($\mathop T\nolimits_4 $) and tri-iodothyronine ($\mathop T\nolimits_3 $). Each one of them is iodinated forms of an amino acid called tyrosine and these two always get stored in the jelly-like semifluid material (colloid) in the lumen of follicles. $\mathop T\nolimits_3 $, is more active and several times more potent when compared to $\mathop T\nolimits_4 $.
From the thyroid gland usually, $\mathop T\nolimits_4 $, is secreted and it gets converted to $\mathop T\nolimits_3 $, in peripheral tissues e.g., Liver. The thyroid is the only endocrine gland which stores its secretory product in large quantity also involves in iodine metabolism. When necessary, the hormones are released from the colloid to the blood.
These have the same and identical effects on the target cells and this is the reason they are together grouped as thyroid hormones (TH). The synthesis of thyroid hormones requires iodine. These hormones are released under the effect of TSH, released by the anterior pituitary.
Functions of Thyroid Hormones (Thyroxine)
1) They control the metabolism of proteins, fats, and carbohydrates and hence regulate the growth of the body tissues
2 ) These hormones play an important role in the regulation of the basal metabolic rate (BMR). Thyroid hormones is helpful to increase the metabolic rate of the body and as a result, they enhance heat production (calorigenic effect) and maintains BMR (basal metabolic rate).
Thyroid hormones also promote the growth of body tissues-both physical growth and the development of mental faculties are stimulated.
They stimulate tissue differentiation. Due to the action they perform, they help to start metamorphosis of tadpoles into adult frogs
To enhance some actions of neurotransmitters: adrenaline and nor-adrenaline.
Maintenance of water and electrolyte balance is also influenced by thyroid hormones.
They support the erythropoiesis, i.e., the process of formation of red blood cells.

Note:
lodine is essential for the normal rate of hormone synthesis in the thyroid. Deficiency or less amount of
iodine in the diet results in the low secretion of thyroid hormone (hypothyroidism) and enlargement of the thyroid gland (simple goiter).