Eugene J. Barrett
The thyroid gland is located in the anterior neck, lying like a small bow tie across the front of the trachea. In adults, the normal thyroid weighs ~20 g. It is composed of left and right lobes and a small connecting branch, or isthmus.
The thyroid gland possesses many features unique among endocrine glands, not the least of which is that it is the only endocrine gland common to both sexes that can be easily seen and palpated in the course of a routine clinical examination. At the biochemical level, the thyroid hormones are the only ones that require an essential trace element, iodine, for the production of active hormone. Another unusual feature of thyroid hormone physiology is that the hormone is stored in an extracellular site within a highly proteinaceous material called thyroid colloid. The major protein within this material is thyroglobulin, which contains—as part of its primary structure—the thyroid hormones tetraiodothyronine (T4 or thyroxine) and triiodothyronine (T3). Thyroglobulin, the sequestered prohormone, is entirely surrounded by thyroid follicular cells, which are responsible for the synthesis of thyroid hormones (Fig. 49-1).
FIGURE 49-1 Structure of the thyroid gland. The thyroid gland is located anterior to the cricoid cartilage in the anterior neck. The gland comprises numerous follicles, which are filled with colloid and lined by follicular cells. These follicular cells are responsible for the trapping of iodine, which they secrete along with thyroglobulin—the major protein of the thyroid colloid—into the lumen of the follicle.
The physiological actions of thyroid hormones also display several unique aspects. Although derived from a large protein (i.e., thyroglobulin), T4 and T3 are not peptides. Unlike peptide hormones or hormones derived from single amino acids (e.g., catecholamines), no cell membrane receptors exist for T4 or T3. Instead, like the steroid hormones, thyroid hormones act principally by binding to nuclear receptors (see pp. 71–72) and regulate the transcription of cell proteins. T4 and T3 act on multiple tissues and are essential for normal development, growth, and metabolism.
The C cells (parafollicular cells) of the thyroid, which are not part of the follicular unit, synthesize another hormone, calcitonin (see Fig. 49-1). Calcitonin may play a role in Ca2+ and phosphate homeostasis, and we discuss its physiology on pages 1067–1068.