An extensive network of fenestrated capillaries derived from the superior and inferior thyroid arteries surrounds the follicles. Blind-ended lymphatic capillaries are present in the interfollicular connective tissue and may also provide a second route for conveying the hormones from the gland.
Thyroid gland function is essential to normal growth and development.
The thyroid gland produces three hormones, each of which is essential to normal metabolism and homeostasis (Table 21.7):  Thyroxine (tetraiodothyronine, T4) and triiodothyronine (T3) are synthesized and secreted by the follicular cells. Both hormones regulate cell and tissue basal metabolism and heat production and influence body growth and development. Secretion of these hormones is regulated by TSH released from the anterior lobe of the pituitary gland.
 Calcitonin (thyrocalcitonin) is synthesized by the parafollicular cells (C cells) and is a physiologic antagonist to parathyroid hormone (PTH). Calcitonin has an important role in regulating serum calcium levels in lower animals; however, its physiological role in humans remains elusive. Calcitonin lowers blood calcium levels by suppressing the
FIGURE 21.15 • Electron micrograph of a parafollicular cell. Cytoplasmic processes of follicular cells (arrows) partially surround the parafollicular cell (PC), which contains numerous electron-dense granules and a prominent Golgi apparatus (G). A basal lamina (BL) is associated with the follicular cells (FC). A portion of the central mass of colloidal material (C) in two adjacent follicles can be seen in the left corners of the micrograph. 12,000. (Courtesy of Dr. Emmanuel-Adrien Nunez.) resorptive action of osteoclasts and promotes calcium deposition in bones by increasing the rate of osteoid calcification. Secretion of calcitonin is regulated directly by blood calcium levels. High levels of calcium stimulate secretion; low levels inhibit it. Secretion of calcitonin is unaffected by the hypothalamus and pituitary gland. Calcitonin is secreted by several endocrine tumors (e.g., a medullary carcinoma of the thyroid); therefore, it is used as a tumor marker to monitor progress of recovery after surgical resection of the tumor. Although calcitonin is used to treat patients with several disorders associated with excess bone resorption (e.g., osteoporosis and Paget’s disease), no clinical disease has been associated with its deficiency or even its absence after total thyroidectomy.
The principal component of colloid is thyroglobulin, an inactive storage form of thyroid hormones.
The principal component of colloid is a large (660 kilodalton) iodinated glycoprotein called thyroglobulin containing about 120 tyrosine residues. Colloid also contains several enzymes and other glycoproteins. It stains with both basic and acidic dyes and is strongly PAS positive. Thyroglobulin is not a hormone. It is an inactive storage form of the thyroid hormones. Active thyroid hormones are liberated from thyroglobulin and released into the fenestrated blood capillaries that surround the follicles only after further cellular processing. The thyroid is unique among endocrine glands because it stores large amounts of its secretory product extracellularly.
Synthesis of thyroid hormone involves several steps.
The synthesis of the two major thyroid hormones, thyroxine (T4) and T3 takes place in the thyroid follicle in a series of discrete steps (Fig. 21.16): 1.