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.

T3 and T4 hormones

The primary function of the thyroid gland is to produce and secrete T4 and T3, which are hormones involved in many aspects of growth, development, and metabolism. T4 and T3 are produced from thyroglobulin attached to iodide. Iodine obtained from the diet is absorbed through the small intestine, converted into iodide, and transported through the blood stream to the thyroid. The iodide absorbed by the thyroid attaches to thyroglobulin and forms monoiodotyrosine (MIT) and diiodotyrosine (DIT). T4 is formed when two DITs join together and T3 is formed when one MIT joins to one DIT. At this point the T3/T4 are still attached to the thyroglobulin. The thyroglobulin containing T4 and T3 is then transported to the center of the follicle where it forms colloid. When there are low levels of T4 and T3 in the blood, the follicular cells are stimulated to ingest colloid, and digest the thyroglobulin. This ultimately results in the release of T4 and T3 into the blood stream.

REGULATION OF T3 AND T4 SYNTHESIS AND RELEASE. Thyroid stimulating hormone (TSH), which is also called thyrotropin, is the main regulator of thyroid hormone synthesis and release. TSH is produced by the pituitary gland. Binding of TSH to receptors on the thyroid gland stimulates the synthesis and release of T4 and T3. High concentrations of TSH result in increased thyroid hormone synthesis and release into the blood stream, and low levels of TSH result in decreased synthesis and decreased release into the blood stream. The amount of TSH secreted is controlled by the thyroid-releasing hormone, which is produced by an organ called the hypothalamus. When the amount of thyroid hormones in the blood exceeds a certain level, the hypothalamus stops secreting thyroid-releasing hormone. This stops the secretion of TSH, which stops the secretion of T3 and T4. This is called a negative feedback loop. When the levels of thyroid hormones in the blood stream decrease to below a predetermined level then the negative feedback is stopped and the secretion of thyroid-releasing hormone resumes. This ultimately results in resumed secretion of T4 and T3. The amount of T4 and T3 produced can also be influenced by dietary factors such as the amount of iodine consumed and the total caloric intake and can also be affected by inhibitory drugs such as the thionamides.

Calcitonin

The thyroid gland also secretes calcitonin. The thyroid's C-cells are stimulated to secrete calcitonin when there is a high concentration of calcium in the blood stream. The function of calcitonin is to inhibit the amount

of resorption of calcium from the bone and to regulate the amount of calcium in the blood stream.