Morphology

The human thyroid gland is located at the base of the neck and wraps around the trachea just below the cricoid cartilage (Fig. 1). The two large lateral lobes that comprise the bulk of the gland lie on either side of the trachea and are connected by a thin isthmus. A third structure, the pyramidal lobe, which may be a remnant of the embryonic thyroglossal duct, is sometimes also seen as a finger-like projection extending headward from the isthmus. The thyroid gland in the normal human being weighs about 20 g but is capable of enormous growth, sometimes achieving a weight of several hundred grams when stimulated intensely over a long period of time. Such enlargement of the thyroid gland, which may be grossly obvious, is called a goiter and is one of the most common manifestations of thyroid disease.

The thyroid gland receives its blood supply through the inferior and superior thyroid arteries, which arise from the external carotid and subclavian arteries. Relative to its weight, the thyroid gland receives a greater flow of blood than most other tissues of the body. Venous drainage is through the paired superior, middle, and inferior thyroid veins into the internal jugular and innominate veins. The gland is also endowed with a rich lymphatic system that may play an important role in delivery of hormone to the general circulation. The thyroid gland also has an abundant supply of sympathetic and parasympathetic nerves. Some studies suggest that sympathetic stimulation or infusion of epinephrine or norepinephrine may increase secretion of thyroid hormone, but it is probably only of minor importance in the overall regulation of thyroid function.

The functional unit of the thyroid gland is the follicle, which is composed of epithelial cells arranged as hollow vesicles of various shapes ranging in size from 0.02-0.3 mm in diameter; it is filled with a glycoprotein colloid called thyroglobulin (Fig. 2). The adult human thyroid gland has about three million follicles. Epithelial cells lining each follicle may be cuboidal or columnar, depending on their functional state, with the height of the epithelium being greatest when its activity is highest. Each follicle is surrounded by a dense capillary network separated from epithelial cells by a well-defined basement membrane. Groups of densely packed follicles are bound together by connective tissue septa

Thyroid cartilage

Cricothyroid ligament Common Carotid Artery

Sternocleidomastoid muscle Cricothyroid muscle Cricoid cartilage

Thyroid gland

Trachea

FIGURE 1 Gross anatomy of the thyroid gland. (From Netter FH, Atlas of human anatomy, 2nd ed., Summit, N.J.: Novartis, 1989. Reprinted with permission from ICON Learning Systems, LLC, a subsidiary of Multimedia USA, Inc.)

to form lobules that receive their blood supply from a single small artery. The functional state of one lobule may differ widely from that of an adjacent lobule. Secretory cells of the thyroid gland are derived embry-ologically and phylogenetically from two sources. Follicular cells, which produce the classical thyroid hormones, thyroxine and triiodothyronine, arise from endoderm of the primitive pharynx. Parafollicular, or C cells, are located between the follicles and produce the polypeptide hormone calcitonin, which is discussed in Chapter 43.

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