The thyroid gland is the body's largest single organ specialized for endocrine hormone production. Its function is to secrete an appropriate amount of the thyroid hormones, primarily 3,5,3′,5′-l-tetraiodothyronine (thyroxine, T4), and a lesser quantity of 3,5,3′-l-triiodothyronine (T3), which arises mainly from the subsequent extrathyroidal deiodination of T4. In target tissues, T3 interacts with nuclear T3 receptors that are, in turn, bound to special nucleotide sequences in the promoter regions of genes that are positively or negatively regulated by thyroid hormone. Among their life-sustaining actions, the thyroid hormones promote normal fetal and childhood growth and central nervous system development; regulate heart rate and myocardial contraction and relaxation; affect gastrointestinal motility and renal water clearance; and modulate the body's energy expenditure, heat generation, weight, and lipid metabolism. In addition, the thyroid contains parafollicular or C cells that produce calcitonin, a 32-amino-acid polypeptide that inhibits bone resorption, but has no apparent physiologic role in humans. However, calcitonin is clinically important as a tumor marker produced by medullary thyroid cancers that arise from these cells (Chapter 8).
The thyroid gland originates in the embryo as a mesodermal invagination in the pharyngeal floor at the foramen cecum, from which it descends anterior to the trachea and bifurcates, forming two lateral lobes, each measuring approximately 4 cm in length, 2 cm in width, and 1 cm in thickness in adulthood. Ectopic thyroid tissue can be present anywhere along or beyond this thyroglossal duct, from the tongue base (lingual thyroid) to the mediastinum. The thyroglossal duct may also give rise to midline cysts lined with squamous epithelium, which can remain asymptomatic, or become infected or give rise to thyroid tumors. The caudal end of the thyroglossal duct forms the pyramidal lobe of the thyroid, which can become palpable in conditions causing diffuse thyroid inflammation or stimulation (Figure 7–1).
Gross anatomy of the human thyroid gland (anterior view).
Upward growth of the thyroid gland is limited by the attachment of the sternothyroid muscle to the thyroid cartilage. However, posterior and downward growth is unimpeded, so that thyroid enlargement, or goiter, frequently extends posteriorly and inferiorly, even into the superior mediastinum (substernal goiter).
The thyroid gland has clinically important anatomical relationships to the recurrent laryngeal nerves, which course behind the gland, and two pairs of parathyroid glands that usually lie behind the upper and middle portions of the thyroid lobes. The thyroid is also draped around the trachea and the posterior margins of its lobes abut the esophagus. All of these structures can be compressed by gland enlargement, invaded by thyroid malignancies, or injured in the course of thyroid surgery (Figure 7–2). Because the posterior thyroid capsule is bound to the pretracheal fascia, the gland normally rises and falls with deglutition, facilitating its inspection and palpation.