Thyroid Gland Health Article

Definition

The thyroid gland is a bilobed organ of the endocrine system located in the front of the neck. It secretes hormones that are involved in human development, growth, and metabolism.

Description

The thyroid gland is a small, butterfly-shaped gland made up of two lobes separated by tissue called the isthmus, which lies across the trachea. The lobes of the thyroid are each approximately 2 inches (5 cm) in length, and the isthmus is approximately 2 inches (5 cm) in width and length and the thyroid gland weighs approximately 1 ounce (28 g). Each lobe of the thyroid gland wraps around and is affixed by fibrous tissue to one side of the trachea. A narrow projection of thyroid tissue, called the pyramidal lobe, is often present and originates at the isthmus and extends up to and lays on the surface of the thyroid cartilage (Adam's apple). The upper projections of the right and left lobes are called the upper poles of the gland while the lower projections of the lobes are called the lower poles. The lobes of the thyroid lie between the larynx and trachea medially and the sternomastoid muscles and carotid sheath laterally. The thyroid gland can be felt through palpitation of the neck, unless the neck is very thick and short or the sternomastoid muscles are very well developed.

A thin capsule of connective tissue surrounds the thyroid and divides it into a cluster of globular sacks called follicles. The gland does not, however, have any true subdivisions, and the follicles are packed together like a bag of berries. The follicles are lined with follicular cells that secrete hormones called thyroxine (T4) and triiodothyronine (T3) and enclose a glutinous material called colloid. Colloid is primarily made of a protein called thyroglobulin that is involved in the formation of T4 and T3. Cells called parafollicluar cells or C-cells, which secrete the hormone calcitonin, are found between the follicles.