The Anatomy of the Thyroid

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The thyroid gland is generally described as a brownish-red gland that is located anterior part in the neck. It is found below the thyroid cartilage, which forms the Adam’s apple and lie against and around the trachea and the larynx. It is also described to be shaped like a butterfly. However, in the field of medicine, they describe the thyroid’s shape to be forming a letter H or a U.

The thyroid is formed by two lobes. The right lobe is called the lobus dexter and the left lobe is called the lobus sinister. Each of these lobes can be 50-60 mm in length. These two lobes are then connected by a median which is referred to as the isthmus.The isthmus has an average length of 12- 15 mm. With all these, the adult thyroid gland can weigh 50-60 grams. However, in women, their thyroid can be heavier and even enlarges during menstruation and when they are pregnant.

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The whole of the thyroid is then covered by a sheath which is fibrous in nature.It is called capsula glandulae thyroidea and it is made of 2 layers, the external and the internal. The outer layer is described to be anteriorly continuous and connected to the lamina pretracheal fascia which then helps form the carotid sheath. The thyroid gland is also covered anteriorly with infrahyoid muscles or the group of four muscles in the anterior part of the neck and on the lateral part would be the sternomastoid muscle.

When it comes to the surface of the thyroid gland, it is covered by the strenothyroid muscle. It is prevented to extend domineeringly under the thyroid muscle by its attachment to the sloping line of the thyroid cartilage. In the extreme anterior side, the strenohyoid and superior belly of the monohyoid muscle is inferiorly overlapped by the sternocledomastoidd muscle’s anterior border. These muscles are referred to in a collective term called strap muscles.

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When it comes to the structure of the thyroid, it is made of layers. Under the middle layer of the deep cervical fascia, there is the thyroid inner true capsule. The capsule extensions are formed by various septae which divides the inner true capsule into lobes and lobules. The lobules are then formed and composed of follicles. The follicles vary in size due to the degree of distention. The surrounding dense plexuses or capillaries, sympathetic nerves and lymphatic vessels also affect the size of the follicles. In general terms, follicles are known to be the structural units of the thyroid gland and it is made of a layer of simple epithelium enclosing a colloid-filled cavity. The colloid is the one that contains the iodinated glycoprotein and iodothyroglobulin which are required for the production of thyroid hormones.

There are 2 types of cells found in this part. Both cells are under the category of Epithelial cells. The first one is called principal cells and they form the colloid. The second type is called parafollicular cells and they lie adjacent to the follicles found within the basal lamina. The parafollicular aids in the production of calcitonin in the thyroid.

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Another description that would suit the thyroid in discussing its anatomy is its being highly vascular, meaning, it is abundant in blood vessels.There are major arteries that are present in the thyroid. There is superior thyroid artery can be found in the anterior branch of the ECA and it descends laterally under the omohyoid and sternohyoid mucles. They also run superficially on the anterior border of the lateral thyroid lobe. It sends a branch deep in the thyroid gland before it curves toward the isthmus where it anastomoses with the collateral artery.The second important artery is the inferior thyroid artery. It ascends vertically and then curves medially to enter the tracheoesophageal groove which is found in the posterior part of the carotid sheath. Its branches then penetrates the posterior area of the lateral lobe.

Aside from the arteries, there are also important veins that are found in the thyroid gland. They provide drainage to the thyroid gland. There are three pair of these veins and they are the superior thyroid vein, the middle thyroid vein and the inferior thyroid vein. The superior thyroid vein can be seen as it ascends along the superior thyroid artery and becomes a branch of the internal jugular vein. The middle thyroid vein follows a horizontal course to the internal jugular vein. The inferior thyroid veins then follow various trails on each side with the right passing the anterior to the innominate artery to the right brachiocephalic vein. On the left side, veinous drainage is found to the left of the brachiocephalic vein. And just like the thyroid lobes, the inferioir veins usually forms a common stem which is called the thyroid ima vein. This then empties into the left brachiocephalic vein.

Along with the veins, there are also lymphatic drainage systems in the thyroid. These flow extensively and multidirectionally. The immediate lymphatic drainage travels to the periglandular nodes. After this, it follows the course to the prelaryngeal, pretracheal nodes which are found along the intermittent laryngeal nerve. Lastly, it crosses the mediastinal lymph nodes.

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