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Embryology

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The thyroid gland develops from a tubular invagination of the embryonic pharynx (the thyroglossal duct), which migrates downward into the neck and develops there into the thyroid gland.

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Thyroid gland ectopia results when there is arrest of this downward migration. The most extreme form is the very rare lingual thyroid, found at the root of the tongue. More frequently, ectopic thyroid tissue in the midline of the neck along the path of descent is present in addition to the normal gland. Rarely, migration may proceed too far, resulting in a gland located in the superior mediastinum.

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Epithelial remnants of the thyroglossal duct may persist in the midline of the neck and may produce cysts, lined by either squamous or respiratory epithelium with thyroid tissue in the wall (thyroglossal duct cyst). Thyroglossal duct cysts are commonly found between the hyoid bone and the isthmus of the thyroid gland. They present in late childhood or early adult life. Infection and abscess formation may occur.

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Structure

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The adult thyroid weighs 20–25 g and is composed of two lateral lobes joined across the midline by an isthmus. A pyramidal lobe of varying size extends upward from the isthmus and represents the point of attachment of the thyroglossal duct. The pyramidal lobe cannot be palpated in a normal gland. The thyroid is firm, reddish-brown, and smooth. It is surrounded by a fibrous capsule that blends with the deep cervical fascia.

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Histologically, the thyroid is composed of closely packed follicles separated by a rich vascular supply and little intervening stroma. The follicles are lined by cuboidal epithelial cells and contain colloid, a proteinaceous material composed mainly of thyroglobulin and stored thyroid hormones.

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Dispersed between the thyroid follicles are the parafollicular or C cells, which secrete calcitonin.

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Synthesis of Thyroid Hormone

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The rate of synthesis of thyroid hormone is controlled by the level of pituitary thyrotropin (thyroid -stimulating hormone (TSH)) in the blood (Figure 58-1). Thyrotropin regulates all of the steps in the synthesis of thyroid hormone. The effect of TSH on the rate of iodide trapping by the thyroid is believed to be the principal factor determining the rate of hormone synthesis by the gland. TSH also induces an increase in the number and size of thyroid follicular cells.

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Figure 58–1.
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Thyroid hormone production and its control. (TRF, thyrotropin-releasing factor; TSH, thyroid-stimulating hormone; MIT, monoiodotyrosine; DIT, diiodotyrosine.)

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After trapping, the iodide is oxidized to iodine in the thyroid cell and is incorporated into tyrosine molecules to form monoiodotyrosine (MIT) and diiodotyrosine (DIT) (Figure 58-1), which are linked with the thyroglobulin molecule in the colloid. These are then coupled enzymatically to form thyroxine (T4) and triiodothyronine (T...

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