The Thyroid Gland
The thyroid gland is an endocrine structure, located in the neck. In this article, we shall look at the anatomical position, vasculature, innervation and any clinical correlations.
The thyroid gland is located in the anterior neck, spanning between the C5 and T1 vertebrae. It is an endocrine gland, divided into two lobes which are connected by an isthmus. It is said to have a butterfly shape.
It lies behind the sternohyoid and sternothyroid muscles, wrapping around the cricoid cartilage and superior tracheal rings. It is inferior to the thyroid cartilage of the larynx. The gland is in the visceral compartment of the neck, along with the trachea, oesophagus and pharynx. The compartment is bound by pretracheal fascia.
During development, the thyroid gland initially forms in the floor of the primitive pharynx, near the base of the tongue. It descends down the neck to lie in its adult anatomical position.
Clinical Relevance: Thyroglossal Cysts
As the thyroid gland descends during development, it moves through a duct called the thyroglossal duct. This duct normally fuses and regresses in the adult.
However, in 50% of individuals, the distal portion of the duct continues as a pyramidal lobe – effectively an extra piece of thyroid tissue. This does not have any clinical consequences.
Other portions of the duct may persist as thryoglossal cysts. These present with a mass in the midline of neck, and can be excised surgically.
The thyroid gland secretes hormones directly into the blood. Therefore it needs to be highly vascularised. Blood supply to the thyroid gland is achieved by two main arteries; the superior and inferior thyroid arteries. These are paired arteries arising on both the left and right.
The superior thyroid artery is the first branch of the external carotid artery. After arising, the artery descends toward the thyroid gland. As a generalisation, it supplies the superior and anterior portions of the gland.
The inferior thyroid artery arises from the thyrocervical trunk (which in turn is a branch of the subclavian artery). The artery travels superomedially to reach the inferior pole of the thyroid. It tends to supply the postero-inferior aspect.
In a small proportion of people (around 10%), there is an additional artery present; the thyroid ima artery. It comes from the brachiocephalic trunk of the arch of aorta, supplying the anterior surface and isthmus.
Venous drainage is carried out by the superior, middle and inferior thyroid veins, which form a venous plexus. The superior and middle veins drain into the internal jugular veins, whereas the inferior drains into the brachiocephalic vein.
The thyroid gland is innervated by branches derived from the sympathetic trunk. However, these nerves do not control endocrine secretion – release of hormones is regulated by pituitary gland.
Clinical Relevance: Recurrent Laryngeal Nerve
The are two recurrent laryngeal nerves; one left and one right. They arise from their respective vagus nerves, and descend into the chest.
In the chest, they hook around the subclavian artery (right RL nerve), or the arch of aorta (left RL nerve). The nerves then ascend back up the neck, running between the trachea and oesophagus. They pass underneath the thyroid gland to innervate the larynx.
During surgery on the thyroid gland, care must be taken not to ligate or damage the recurrent laryngeal nerves.