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THE THYROID GLAND

EMBRYOLOGY & ANATOMY

The thyroid gland develops as a median endodermal downgrowth from the first and second pharyngeal pouches (Figure 17–1). During its caudal migration, 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, extending cranially from the isthmus, is frequently present. Agenesis of one thyroid lobe, almost always the left, may occur.

Figure 17–1.

Thyroid anatomy. The recurrent laryngeal nerve runs in the tracheoesophageal groove on the left and has a slightly more oblique course on the right before it enters the larynx just posterior to the cricothyroid muscle at the level of the cricoid cartilage.

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. Venous drainage is via the superior, middle, and inferior thyroid veins.

PHYSIOLOGY

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. T4 is the dominant hormone secreted by the thyroid, and extrathyroidal conversion of T4 to T3 accounts for the majority of circulating T3.

Thyroid function is regulated by a feedback mechanism that involves the hypothalamus and pituitary. Thyrotropin-releasing factor (TRF) is formed in the hypothalamus and stimulates the release of the thyroid-stimulating hormone (TSH) thyrotropin from the pituitary. Thyrotropin binds to TSH receptors on the thyroid plasma membrane, stimulating thyroid cellular function and thyroid growth.

EVALUATION OF THE THYROID

In a patient presenting with thyroid disease, a thorough history and physical examination of the gland are critical and are complemented by appropriate laboratory tests and imaging. The history should question both systemic and local symptoms. Family history of thyroid dysfunction or malignancy is also ...

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