THE THYROID GLAND
In most mammals the thyroid gland is located caudal to the trachea at the level of the first or second tracheal ring. I he thyroid gland is composed of two lobes lying on either side of the trachea and connected by a narrow piece of tissue called the isthmus.
The thyroid gland is the most important endocrine gland for metabolic regulation. The glandular tissue has cells formed in a circular arrangement called a follicle (Figure 34-1). The follicles are filled with a homogeneous-staining substance called colloid, which is the main storage form of the thyroid hormones. The follicular cells are Cuboidal when the secretion is
FIGURE 34-1 Histological features of the normal thyroid gland of the rat. All normal thyroid glands are structurally similar, although slight variations occur with age, diet, habitation, and sexual status (neutered or intact). The normal animals of the colony to which this rat belonged were maintained on a high-protein ration, which probably accounts for the slight hypertrophic condition of the secretory epithelium. (FromTurner CD, Bagnara JT: General endocrinology, ed 6, Philadelphia, 1976, Saunders.)
basal and are elongated when the cells are stimulated to release hormone. Another important endocrine cell, the parafollicular cell, or C cell, is located outside the follicles. This cell secretes calcitonin, a hormone important for the regulation of calcium. The activity of this hormone is discussed in the section on calcium metabolism.