Review of Medical Physiology - William F. Ganong 2002

The Endocrine System, Metabolism, and Reproduction
The Thyroid Gland
Anatomical Structure

Thyroid tissue is present in all vertebrates. In mammals, the Thyroid Gland develops from an evagination of the floor of the Pharynx. The thyroglossal duct, which traces the developmental descent of the thyroid from the Tongue to the neck, can sometimes still be found in adults. The two lateral lobes of the thyroid are connected by a bridge of tissue known as the thyroid isthmus; anterior to it, a pyramidal lobe often extends upward from the isthmus (Fig. 18-1). The gland is highly vascularized, boasting one of the highest rates of Blood flow per gram of tissue of any organ in The Human Body.

The thyroid gland is composed of numerous acini (follicles). Each spherical follicle is surrounded by a single layer of Cells and filled with a pink-staining proteinaceous substance called colloid. When the gland is inactive, colloid accumulates in large amounts; the follicles become distended, and the lining cells flatten. Conversely, when the gland is active, the follicles decrease in size, the cells become cuboidal or columnar, and the margins of the colloid appear scalloped, forming numerous small resorption lacunae (Fig. 18-2). Microvilli project from the apical surface of the thyroid cells into the colloid, accompanied by intracellular canaliculi. The cells exhibit a prominent Endoplasmic reticulum—a feature characteristic of most glandular cells—along with secretory droplets of thyroglobulin (Fig. 18-3). Every thyroid Cell is anchored to a basement membrane, which separates the cells from adjacent capillaries. These capillaries are fenestrated (see Fig. 18-3), much like the capillaries found in other Endocrine glands (see Chapter 30).

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Fig. 18-1. The human thyroid gland.

Fig. 18-2. Histology of the thyroid gland. Note the small scalloped resorption lacunae bordering the cells within the colloid of an active gland.