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The normal thyroid gland is located anterior to the trachea and midway between the apex of the thyroid cartilage and the suprasternal notch (Figure 42–1). Important neighboring posterior structures include the four parathyroid glands situated behind the upper and middle thyroid lobes, and the recurrent laryngeal nerves coursing along the trachea. The thyroid consists of two pear-shaped lobes connected by an isthmus. The typical dimensions of the lobes are 2.5–4.0 cm in length, 1.5–2.0 cm in width, and 1.0–1.5 cm in thickness. Also, in about 50% of patients, a small pyramidal lobe is present at the isthmus or adjacent part of the lobes. The functional unit of the thyroid is the follicle consisting of a central collection of colloidal material (thyroglobulin) surrounded by a single layer of polarized epithelial cells.
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A normal thyroid gland weighs approximately 10–20 g, depending on dietary iodine intake, age, and weight. The thyroid gland usually grows posteriorly and inferiorly, since it is limited from upward extension by the sternothyroid muscle. In large multinodular goiters, substernal extension is not uncommon.
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The thyroid gland has a rich blood supply, derived from the superior, inferior, and the small inferior ima artery (Figure 42–2). Venous flow returns via multiple surface veins draining into the superior, lateral, and inferior thyroid veins.
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Thyroid Hormone Synthesis
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The essential steps to thyroid hormone production are as follows (Figure 42–3).
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Active Uptake of Iodide (I−) into the Thyroid Cell (Trapping)
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Iodide is actively transported across the basal membrane of the thyroid cell by membrane-bound sodium-iodide symporters (NIS). The iodide concentration inside the cell is about 30 to 40 times greater than in the plasma. NIS action is stimulated by thyroid-stimulating hormone (TSH).
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Oxidation of Iodide and Iodination of Tyrosyl Residues in Thyroglobulin (Organification)
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Iodide entering the thyroid cell is oxidized by locally produced hydrogen peroxide to an active iodide intermediate which then covalently binds to the tyrosyl residues in thyroglobulin at the apical-colloid border. The thyroglobulin molecule is a dimer of two identical chains and about 1/3 of the tyrosyl residues of the molecule undergoes iodination. Thyroid peroxidase (TPO) catalyzes both iodide oxidation ...