Skip to Main Content

++

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.

++
Figure 42–1.
Graphic Jump Location

Gross anatomy of the thyroid gland. (Reproduced, with permission, from Greenspan FS: Basic and Clinical Endocrinology. Originally published by Appleton & Lange. New York: The McGraw-Hill Companies, 1983.)

++

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.

++

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.

++
Figure 42–2.
Graphic Jump Location

Arterial and venous blood supply of the thyroid gland. (Reproduced, with permission, from Lindner HH: Clinical Anatomy. Originally published by Appleton & Lange. New York: The McGraw-Hill Companies, 1989.)

++

Thyroid Hormone Synthesis

++

The essential steps to thyroid hormone production are as follows (Figure 42–3).

++
++

Active Uptake of Iodide (I) into the Thyroid Cell (Trapping)

++

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).

++

Oxidation of Iodide and Iodination of Tyrosyl Residues in Thyroglobulin (Organification)

++

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 ...

Want remote access to your institution's subscription?

Sign in to your MyAccess profile while you are actively authenticated on this site via your institution (you will be able to verify this by looking at the top right corner of the screen - if you see your institution's name, you are authenticated). Once logged in to your MyAccess profile, you will be able to access your institution's subscription for 90 days from any location. You must be logged in while authenticated at least once every 90 days to maintain this remote access.

Ok

About MyAccess

If your institution subscribes to this resource, and you don't have a MyAccess profile, please contact your library's reference desk for information on how to gain access to this resource from off-campus.

Subscription Options

AccessSurgery Full Site: One-Year Subscription

Connect to the full suite of AccessSurgery content and resources including more than 160 instructional videos, 16,000+ high-quality images, interactive board review, 20+ textbooks, and more.

$995 USD
Buy Now

Pay Per View: Timed Access to all of AccessSurgery

24 Hour Subscription $34.95

Buy Now

48 Hour Subscription $54.95

Buy Now

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.