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The main anlage of the thyroid gland develops as a median endodermal downgrowth from the first and second pharyngeal pouches (Figure 16–1). During its migration caudally, it contacts the ultimobranchial bodies developing from the fourth pharyngeal pouches. When it reaches the position it occupies in the adult, with the isthmus situated just below the cricoid cartilage, the thyroid divides into two lobes. The site from which it originated persists as the foramen cecum at the base of the tongue. The path the gland follows may result in thyroglossal remnants (cysts) or ectopic thyroid tissue (lingual thyroid). A pyramidal lobe is frequently present. Agenesis of one thyroid lobe, almost always the left, may occur.
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The normal thyroid weighs 15-25 g and is attached to the trachea by loose connective tissue. It is a highly vascularized organ that derives its blood supply principally from the superior and inferior thyroid arteries. A thyroid ima artery may also be present.
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The function of the thyroid gland is to synthesize, store, and secrete the hormones thyroxine (T4) and triiodothyronine (T3). Iodide is absorbed from the gastrointestinal tract and actively trapped by the acinar cells of the thyroid gland. It is then oxidized and combined with tyrosine in thyroglobulin to form monoiodotyrosine (MIT) and diiodotyrosine (DIT). These are coupled to form the active hormones T4 and T3, which initially are stored in the colloid of the gland. Following hydrolysis of the thyroglobulin, T4 and T3 are secreted into the plasma, becoming almost instantaneously bound to plasma proteins. Most T3 in euthyroid individuals, however, is produced by extrathyroidal conversion of T4 to T3.
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The function of the thyroid gland is regulated by a feedback mechanism that involves the hypothalamus and pituitary. Thyrotropin-releasing factor (TRF), a tripeptide amide, is formed in the hypothalamus and stimulates the release of the thyroid-stimulating hormone (TSH) thyrotropin, a glycoprotein, from the pituitary. Thyrotropin binds to TSH receptors on the thyroid plasma membrane, stimulating increased adenylyl cyclase activity; this increases cyclic adenosine monophosphate (cAMP) production and thyroid cellular function. Thyrotropin also stimulates the phosphoinositide pathway and—along with cAMP—stimulates thyroid growth.
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EVALUATION OF THE THYROID
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In a patient with enlargement of the thyroid (goiter), the history (including local and systemic systems and family history) and examination of the gland are most important and are complemented by the selective use of thyroid function ...