The thyroid is located in the mid-line of the neck, anterior to the trachea and inferior to the larynx.
It is found within the pre-tracheal fascia, one of the fascial compartments of the neck. It is a thin fascia in the anterior part of the neck and is composed of two parts:
The thyroid gland is comprised of two lateral lobes connected by a central isthmus. It is surrounded by a fibrous capsule and located at the level of vertebrae C5-T1.
The pyramidal lobe, an embryological remnant from the descent of the thyroid, typically projects upwards from the isthmus, however, there is a wide degree of variation between individuals.
The primary function of the thyroid gland is to produce thyroid hormones (T3 and T4) following stimulation by thyroid-stimulating hormone (TSH).
The thyroid gland is surrounded by a number of important structures.
The thyroid cartilage is one of the three unpaired laryngeal cartilages (the others being the cricoid cartilage and epiglottis). It is formed by hyaline cartilage and is found above the thyroid gland, sitting at the level of vertebrae C4-5.
It forms a median projection, more prominent in males, termed the Adam's apple. Superior to this projection is the superior thyroid notch. The cartilage has both superior and inferior horns.
The cricoid cartilage sits at the level of vertebrae C6. It is the only complete ring of cartilage in the trachea. It is composed of a posterior component termed the lamina and an anterior component termed the arch. It is attached to the first tracheal ring by the cricotracheal ligament.
This ligament connects the inferior border of the thyroid cartilage with the superior border of the cricoid cartilage. An incision is made through the medium cricothyroid ligament to establish an emergency airway - a cricothyroidotomy.
The parathyroid glands (typically four) are located posterior to the thyroid gland. It is important to note there is a great deal of variety in both the location and number of glands.
They are responsible for the release of parathyroid hormone, a key part of the calcium homeostasis pathways. Due to their location, they are frequently removed, injured or devascularised during a total thyroidectomy, typically resulting in a transient hypocalcaemia.
A rich blood supply is received from the external carotid artery and the thyrocervical trunk.
The thyroid gland is drained by three pairs of veins.
The recurrent laryngeal nerve is a branch of the vagus nerve (cranial nerve X).
The right and left recurrent laryngeal nerves branch off the left and right vagus nerves at different levels. The right side branches of at the level of the subclavian artery whilst the left side branches at the level of the arch of the aorta.
They follow a ‘recurrent’ path returning back upwards through a groove between the trachea and oesophagus. This brings them close to the thyroid gland. The nerve may be damaged during thyroid surgery, typically resulting in vocal cord paralysis on the affected side and a hoarse voice.
The recurrent laryngeal nerve has a number of functions:
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