The Thyroid Gland

The Thyroid Gland

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The larynx | Main Anatomy Index | Neck Index

Last updated 1 April 2006

The Thyroid Gland

The thyroid gland consists of left and right lobes that are united by a narrow isthmus that extends across the trachea (anterior to the 2^nd and 3^rd tracheal cartilages).
It is the largest of all endocrine glands and clasps the anterior and lateral surfaces of the pharynx, larynx, oesophagus and trachea like a shield.

The thyroid gland lies deep to the sternothyroid and sternohyoid muscles.

Each pear-shaped lobe extends inferior on each side of the trachea, often to the level of the sixth tracheal cartilage posteriorly, and extends along the sides of the oesophagus.

A conical pyramidal lobe is present in some people, which ascends from the isthmus of the thyroid gland, usually from the left side, toward the hyoid bone is present in some people.

A thin fibrous capsule of connective tissue surrounds the thyroid gland.
External to this, there is a false capsule formed a sheath of pretracheal fascia.

The gland is attached to the arch of the cricoid cartilage and to the oblique line of the thyroid cartilage.

Arterial Supply of the Thyroid Gland (p. 817)

This gland receives its blood from two large arteries: the superior and inferior thyroid arteries.
These vessels lie between the capsule and pretracheal fascia (false capsule).

All the thyroid arteries anastomose with each other on or in the substance of the thyroid gland.
There is little anastomosis across the median plane, however, expect for branches of the superior thyroid artery.

The Superior Thyroid Artery (p. 817)

This is the first branch of the external carotid artery.
It descends to the superior pole of the gland, pierces the pretracheal fascia, and then divides into two or three branches.

The Inferior Thyroid Artery (pp. 817, 820)

This is a branch of the thyrocervical trunk.
It runs superomedially posterior to the carotid sheath to reach the posterior aspect of the gland.
It divides into several branches, which pierce the pretracheal fascia to supply the inferior pole of the thyroid gland.
The inferior thyroid artery has an intimate relationship with the recurrent laryngeal nerve.

Venous Drainage of the Thyroid Gland (p. 820)

Usually there are three pairs of veins that drain the venous plexus on the anterior surface of the gland.

The superior thyroid veins drain its superior poles.
The middle thyroid veins drain its lateral parts.
The inferior thyroid veins drain its inferior poles.

The superior and middle thyroid veins empty into the internal jugular veins.
The inferior thyroid veins drain into the brachiocephalic veins. They often unite to form a single vein that drains into one of the brachiocephalic veins.

Lymphatic Drainage of the Thyroid Gland (p. 820)

Lymph vessels run in the interlobular connective tissue, often around the arteries.
They communicate with the capsular network of the lymph vessels.
From here, vessels pass to the prelaryngeal lymph nodes and to the pretracheal and paratracheal lymph nodes.

Laterally, lymph vessels located along the superior thyroid veins pass to the inferior deep cervical lymph nodes.
Some lymph vessels may drain into the brachiocephalic lymph nodes or empty directly into the thoracic duct.

Nerve Supply of the Thyroid Gland (p. 820)

These nerves are derived from the superior, middle and inferior cervical sympathetic ganglia.
They reach the thyroid gland through the cardiac and laryngeal branches of the vagus nerve, which runs along the arteries supplying the gland.
These postganglionic fibres are vasomotor and indirectly affect the gland through its blood supply.