The thyroid gland is highly vascular gland. Surrounded by a fibrous capsule. It consist of two lobes lying on either side if the trachea. The lobes are joined together by an isthmus lying in front of trachea. The lobes are joined together by an isthmus lying in front of trachea. The lobes are conical in shape about 5 cm long and 3cm wide and have upper and lower poles. It receives plentiful blood supply from superior thyroid artery which is branch from the external carotid artery and interior carotid a branch from the subclavian artery. The parathyroid glands lie in the posterior surface of each lobe and are embedded in the thyroid tissue.
Structure of Thyroid Gland
Microscopically the thyroid glands contain two types of hormone producing cells.
- The Follicular cells which produce thyroid hormones i.e. thyroxin and Tri idothyronine
- The C or clear cells which produces thyrocalcitonin.
The structure units of the thyroid glands are small sacs or bags known as follicle between the follicles there us a network of reticular fibrous, capillaries, lymphatic and nerves. Each follicle are formed by a single layer of epithelial cells/Follicular cells. That surrounded by a cavity which contains a secretory product known as colloid. The Colloid is a viscous homogeneous fluid, consisting mainly of a protein Iodine complex called hemoglobin which is produce d by the follicular cells. The colloid also contains the type thyroidhormones in the storage from and is made up of proteins, thyroid hormones. The triiodothyronine (T3) and Thyroxin (T4). When these hormones are needed by the body an enzyme (protein splitting enzymes) is the colloid splits the thyroglobin molecules are taken up by the follicular cells for entry into the blood steam.
On entering the blood stream, thyroxin and Triiodothyronine (T 3 and T 4). Combined with a globing fractions of plasma protein and are transported to all parts of the body as ’ protein — bound’ iodine compounds. Iodine is essential for the formation of thyroid hormone T 3 and T 4 both the thyroid hormone contains a high promotion of iodine which the thyroid gland is obtained from food especially sea food and water an iodine deficiency will lead into enlargement of the thyroid gland to form a goiter.
Control of Secretion
Thyroid stimulating hormone from anterior pituitary controls the growth and secondary activity of the thyroid gland. It increases the size and no of follicular cells . TSH also promote the breakdown of thyroglobin in the follicles with the resultant release of triiodothyronine an thyroxin.
The secretion of thyroid stimulating hormones by the anterior pituitary is regulated by the concentration of triiodothyronine or Thyroxin in the circulating blood. When the concentration of this hormone is decreased the pituitary cells are stimulated by a way hypothalamus to secretion .TSH on the hand and increase in the amount of these hormones in the blood leads . to inhibit the production of thyroid stimulating hormones.
Functions of Thyroid Gland
- Tri iodothyronine and Thyroxin controls the metabolic rate of the body.
- It raises the O2 consumption and increase the heat production.
- They are essential for normal growth and development of the body including mental development and attainment of sexual maturation.
- They have under spread effects on all aspects of metabolism such as promoting protein synthesis maintaing normal blood levels of cholesterol and fatty acid increasing the rate of glucose . Absorption and utilisation.
- They assist in maintaing fluid and electrolyte balance .
- It also affects the irritability of central nervous system.
- If plays a part in keeping the skin and hair in a good condition
- Storage of iodine.