Chapter 1: What Is the Thyroid Gland?

The thyroid gland is an endocrine gland located in the neck. Its functions are production, storage, and secretion of two thyroid hormones-thyroxine (T 4) and triiodothyronine (T3). Hormones are chemicals that travel through the blood to every part of the body; thyroid hormones regulate the body's use of energy.

Endocrine glands secrete (release) hormones into the bloodstream. Examples of endocrine glands, in addition to the thyroid gland, are the pituitary gland, parathyroid glands, adrenal glands, ovaries, testes, and those parts of the pancreas that make insulin. Endocrine glands are not to be confused with either sweat glands or with "swollen glands"-a term that people sometimes use to describe swelling of lymph nodes. Doctors who care for patients with disorders of the endocrine glands are called endocrinologists.

The word "thyroid" is derived from the Greek thyreoeides, which means shield-shaped. Dr. Thomas Wharton gave the thyroid gland its name in 1656 because of its proximity to the thyroid cartilage, which is shaped like a shield. The thyroid cartilage, commonly called the "Adam's apple," is more prominent in men than in women.

Thyroid diseases may be viewed as disorders of structure and disorders of function. Since disorders of structure and function frequently overlap, and since patients may have more than one thyroid problem, this distinction is somewhat artificial; nonetheless, it will simplify under­standing what is, and is not, thyroid disease.