Hypothyroidism / The Thyroid Gland

What is it?

All animal life requires oxygen for sustenance, and the human species is no exception. Oxygen drives the basic metabolic processes that permit growth, development, reproduction, physical movement, and constant body temperature. The complex of chemical interactions necessary to sustain these processes is called metabolism, and the prime, overall regulators of metabolism are the thyroid hormones.

Anatomy

The thyroid gland is located in the anterior part of the neck in the midline. It consists of two lateral lobes lying on each side of the thyroid cartilage (Adam's apple) and connected by a band of tissue called the isthmus.

It is one of the larger endocrine glands, and its capacity to grow is phenomenal. Any enlargement of the thyroid, regardless of cause, is called a goitre. The thyroid arises in the embryo from a downward outpouching of the floor of the fetal pharynx, and a persisting remnant of this migration is known as a thyroglossal duct.

If viewed under a three-dimensional microscope, the resting thyroid is seen as a collection of small, generally globular sacs, called follicles, filled with the prohormone thyroglobulin. The cells lining these globules are called follicular cells, and it is their function to synthesize thyroid hormones as part of the prohormone thyroglobulin and either to secrete them directly into the circulation or store them within the follicles. When the individual's requirement for thyroid hormone increases, thyroglobulin is split into its component parts, and the thyroid hormone thus released passes through the follicular cells to enter the circulation. Nestled in the spaces between the follicles are parafollicular cells. These, in essence, form a separate endocrine organ. They have an entirely distinct embryological origin, and they are not embedded in the substance of the thyroid gland, in many species other than man (see the parathyroid glands: calcitonin).

Thyroid Functions

Biochemistry

The thyroid hormones are not proteins; rather, they are modifications, called thyronines, of an amino acid, tyrosine. Thyroid hormones are heavily laden with iodine. The major active thyroid hormones are thyroxine (T4) and triiodothyronine (T3). Even though the thyroid gland manufactures considerably more T4 than T3, T3 is roughly 2 1/2 times more potent than T4. Indeed, in many ways, T4 serves as an additional, circulating depot for T3 in that when T4 leaves the circulation and travels through the cytoplasm to the nucleus of the target cell, its action at that site is preceded or accompanied by its conversion to T3.

Most of the T4 and T3 secreted by the thyroid is bound to special proteins (thyroxine-binding globulin [TBG] and prealbumins) in the serum, although small amounts of these hormones travel freely in the serum and are readily taken up by tissues to be replenished instantaneously from the T4 that had been attached to the binding proteins.

Essentially all the cells in the body are target cells of thyroid hormones. The major function of the thyroid hormones is to stimulate the synthesis of protein once they have entered the cell nucleus. Another important function is to stimulate the activity of the cell's mitochondria. These intracellular organelles are the sites at which there is a controlled exchange of energy. Some energy is conserved for the body's functionings, while the remainder is dissipated as heat. The proportion of energy devoted to each of these processes is controlled by the thyroid hormones. There are other intracellular thyroid hormone functions that are not well understood, but it is clear that thyroid hormones modulate protein, carbohydrate, fat, and vitamin metabolism, as well as the generation of body heat. Thyroid hormones also modify the activity of the autonomic nervous system.