Anatomy of the thyroid and parathyroid glands

The cervical viscera are composed of three layers named after their primary function. The most superficial layer is the endocrine layer which contains the thyroid and parathyroid glands, then there’s the respiratory layer which contains the larynx and trachea and finally, the deepest layer is the alimentary layer which contains the pharynx and esophagus.

Let’s focus on the endocrine layer. The thyroid gland is the largest endocrine gland in the body and produces two hormones: thyroid hormone and calcitonin.

It’s located anteriorly in the neck at the level of the C5 to T1 vertebrae and deep to the sternothyroid and sternohyoid muscles.

It consists of a right and left lobe, located anterolateral to the larynx and trachea. These lobes are united over the trachea by a thin isthmus, usually anterior to the second and third tracheal rings, giving it an H-shaped appearance.

In some people, there is a third lobe called the pyramidal lobe which usually arises from the isthmus and extends up to the hyoid bone.

Now, the thyroid gland is surrounded by a thin fibrous capsule which sends septa deeply into the gland, and this capsule is attached to the cricoid cartilage and the superior tracheal rings by dense connective tissue.External to the capsule, the visceral portion of the pretracheal layer of deep cervical fascia forms a loose sheath, and between this loose sheath and the fibrous capsule, there’s the superior and inferior thyroid arteries which supply the gland.

The superior thyroid arteries are usually the first branches of the external carotid arteries and they descend to the superior poles of the gland, then pierce the pretracheal layer of deep cervical fascia and finally divide into anterior and posterior branches which supply mainly the anterosuperior aspect of the gland.

The inferior thyroid arteries are the largest branches of the thyrocervical trunks and they run superomedially posterior to the carotid sheaths to reach the posterior aspect of the gland where they divide into several branches that pierce the pretracheal layer of deep cervical fascia and supply the posteroinferior aspect of the gland, including the inferior poles of the gland.

In approximately 10% of people, there’s an additional small artery called thyroid ima artery which can arise from different locations such as the brachiocephalic trunk, the arch of aorta, the right common carotid artery, subclavian artery or internal thoracic artery.

When it’s present, this artery ascends on the anterior surface of the trachea, to which it supplies small branches, and then continues to supply the isthmus of the gland.

The venous drainage of the thyroid gland is through three pairs of thyroid veins: superior thyroid veins which accompany the superior thyroid arteries and drain the superior poles of the gland, middle thyroid veins which run parallel with the inferior thyroid arteries and drain the middle of the lobes, and the inferior thyroid veins which drain the inferior poles of the gland.

That’s easy to remember, right? These three pairs of thyroid veins usually form a thyroid plexus of veins on the anterior surface of the gland anterior to the trachea.

The superior and middle thyroid veins drain into the internal jugular vein and the inferior thyroid veins drain into the brachiocephalic veins.

The innervation of the thyroid gland is primarily provided by sympathetic nerves derived from the superior, middle and inferior cervical ganglia that reach the gland through the cardiac plexus: the superior and inferior thyroid periarterial plexuses that accompany the thyroid arteries.

These fibers are vasomotor, causing constriction of the blood vessels. Now, the parathyroid glands are small flattened, oval endocrine glands which secrete a hormone called parathyroid hormone, or PTH.

These glands are usually located on the medial half of the posterior surface of each lobe of the thyroid gland inside its fibrous sheath.

There are typically four glands but this can vary. The superior parathyroid glands are located at the level of the inferior border of the cricoid cartilage and the inferior parathyroid glands are located near the inferior poles of the thyroid gland; but again, this may vary.

The parathyroid glands are supplied by branches of the inferior thyroid artery since this artery is the main blood supplier of the posterior aspect of the thyroid gland.

However, they may also be supplied by branches from superior thyroid arteries, thyroid ima artery or laryngeal, tracheal and esophageal arteries.

The venous drainage is provided by the parathyroid veins which drain into the superior, middle and inferior thyroid veins.

The parathyroid glands are innervated by the thyroid branches of the cervical sympathetic ganglia. Again, these nerves are vasomotor, causing vasoconstriction, and are not involved in the secretion of hormones.##SummaryAll right, as a quick recap.

The cervical viscera are arranged in three layers: endocrine layer, respiratory layer and alimentary layer. The endocrine layer contains the thyroid gland and the parathyroid glands.

The thyroid gland consists of two lobes: right lobe and left lobe which are located anterolateral to the larynx and trachea and they are united over the trachea by a thin isthmus which gives it an H-shaped appearance.

In some people, there is a third lobe called the pyramidal lobe which usually arises from the isthmus and extends up to the hyoid bone.

On the posterior aspect of the thyroid there are four small endocrine glands called the parathyroid glands.