The thyroid gland lies in the lower part of the neck. It is a highly vascular gland, brownish red in colour and surrounded by a fibrous capsule. It consists of two lobes lying one on either side of the trachea and joined together by a strip of thyroid tissue called the isthmus or the thyroid. The lobes are roughly conical in shape with the apex at the upper narrow part and base, the lower broader part.

This gland is composed of number of vesicles formed by secreting cells which secrete a sticky fluid, the colloid of the thyroid which contains an iodine compound. The active principle of this compound is hormones thyroxin and triiodothyronine. The secretion of thyroxin is regulated by the thyroid stimulating hormone (TSH) secreted by the anterior lobe of the pituitary gland.

The main functions of the thyroid gland are:

1. It exercises control over the general metabolism of the body.


2. It affects the irritability of the nervous system or associated with nerve stability.

3. It plays a part in keeping the skin and hair in good condition.

4. It cooperates with other ductless glands in the general endocrine balance of the body.

5. It has an effect on body growth and mental development in infancy.


6. It controls the utilisation of oxygen in the body; thereby have a controlling influence on the basal metabolic rate.

7. It stimulates the absorption of carbohydrate from the small intestine.

8. It influences heat production during the catabolism of nutrient materials in the cells.

9. Storage of iodine.


Over Secretion (Hyper secretion)

When there is over secretion of thyroxin, there is enlargement of the gland which is called goiter, and the disease produced is known as exophthalmia goiter of Grave’s disease. It is characterized by protrusion of eyeballs, swelling of thyroid, rapid, pulse, increase sweating, general nervousness, tremor and the metabolic rate is increased.

Under-secretion (Hypo secretion)

Deficiency of the secretion during infancy produces a condition known as cretinism, in which mental and physical growth are retarded. In adults, thyroid deficiency produces myxoedema in which the general metabolic processes slows down, there is tendency to gain weight, movements become lethargic, there is slowness of mind and speech, skin becomes thickened and dry; the hair falls out and gets thin.