Thyroid gland (glandula thyreoeidea) is an endocrine gland, an organ that releases hormones into the blood, in the lower front of the neck. Its lobes lie beside the lower larynx, or voice box, and the upper trachea, or windpipe. Its hormones influence metabolism, the chemical processes through which cells use energy and materials.
Where is the thyroid gland located?
The thyroid gland occupies the anterior neck, meaning the region towards the front. Its right and left lobes lie on either side of the lower larynx and upper trachea. The isthmus, a bridge of gland tissue, usually joins the lobes across the front of the upper trachea.
The gland is enclosed by a thin fibrous capsule, a covering of connective tissue. Outside this lies a sheath derived from the visceral part of the pretracheal fascia, a connective tissue layer that invests the thyroid gland, trachea and oesophagus. Attachments to the larynx and trachea help the gland move with these structures during swallowing.
The larynx and trachea belong to the respiratory system and provide the central landmarks for locating the gland. The thyroid is outside their airway, not within it.
What structures surround the thyroid gland?
The sternohyoid and sternothyroid muscles cover much of the gland anteriorly. These strap-shaped neck muscles are part of the muscular system and act on the hyoid bone, a bone above the larynx, or on the larynx itself. The sternocleidomastoid muscle overlaps the gland laterally, towards the side of the neck.
The medial surface, facing towards the centre, relates to the larynx, trachea, pharynx and oesophagus. The pharynx is the muscular passage behind the nose and mouth; the oesophagus carries swallowed material towards the stomach. The oesophagus is a particularly important neighbour of the left lobe.
Behind and lateral to each lobe lies the carotid sheath, a connective tissue sleeve containing the common carotid artery, internal jugular vein and vagus nerve. The parathyroid glands, small endocrine glands involved in calcium regulation, usually lie against the posterior aspect of the thyroid. They are separate organs, not parts of the thyroid gland.
What are the parts of the thyroid gland?
Each lateral lobe has an upper apex, meaning its pointed upper end, and a broader lower base. Its surfaces face forwards and laterally, medially, and backwards and laterally. The posterior border is closely related to the parathyroid glands and vessels supplying the thyroid.
| Part | Position | Anatomical feature |
|---|---|---|
| Right and left lobes | Beside the lower larynx and upper trachea | Form the main bulk of the gland |
| Isthmus | Across the front of the upper trachea | Usually connects the lateral lobes |
| Pyramidal lobe | Extends upwards from the isthmus or an adjacent lobe | A variable extension of thyroid tissue |
The pyramidal lobe is not always present. When present, it reflects the gland’s developmental descent from the tongue region into the neck. The isthmus can also vary in form and may be absent.
How is the thyroid gland organised internally?
The thyroid belongs to the endocrine system, whose glands release chemical messengers called hormones into the circulation. Its capsule sends connective tissue partitions into the gland, supporting its vessels and dividing the tissue into smaller groups.
The basic microscopic units are follicles: small rounded spaces lined by follicular cells. Each follicle contains colloid, a protein-rich material that stores substances used to make thyroid hormones. Follicular cells produce and release these hormones.
Parafollicular cells, also called C cells, lie among the follicular cells or between follicles. They produce calcitonin, a hormone involved in calcium regulation. A close network of capillaries, the smallest blood vessels, surrounds the follicles and receives hormones released by the cells.
What supplies blood to the thyroid and drains it?
The cardiovascular system supplies the gland through arteries, vessels carrying blood away from the heart. The superior thyroid artery usually arises from the external carotid artery and approaches the upper part of each lobe. The inferior thyroid artery usually arises from the thyrocervical trunk, a branch of the subclavian artery, and approaches the gland posteriorly.
Their branches communicate within and around the gland. A variable thyroid ima artery may provide an additional supply, usually approaching the isthmus from below.
Veins, which return blood towards the heart, form a network around the gland. The superior and middle thyroid veins drain into the internal jugular veins. The inferior thyroid veins drain into the brachiocephalic veins at the root of the neck.
The lymphatic system carries tissue fluid away through lymph vessels. Thyroid lymph drains to prelaryngeal, pretracheal and paratracheal lymph nodes, located respectively in front of the larynx, in front of the trachea and beside the trachea. Drainage continues to the deep cervical lymph nodes along the internal jugular veins.
What is the thyroid’s nerve supply and function?
Fibres from the nervous system reach the thyroid through autonomic nerves, which regulate involuntary activity. Sympathetic fibres arise through the cervical sympathetic ganglia, collections of nerve cell bodies in the neck, and travel along arteries. They mainly regulate blood vessel tone rather than directly controlling thyroid hormone release.
The recurrent laryngeal nerves usually ascend in or near the groove between the trachea and oesophagus and pass near the posterior thyroid, although their course varies, particularly on the right. The external branch of the superior laryngeal nerve lies close to the superior thyroid vessels. These nerves supply laryngeal structures rather than controlling thyroid hormone production.
Thyroid hormones support metabolic activity, growth and development. Their release is regulated mainly by thyroid-stimulating hormone from the pituitary gland, an endocrine gland at the base of the brain. Calcitonin contributes to calcium regulation by reducing bone breakdown.
How can I find the thyroid gland in the 3D model?
Rotate the 3D model and zoom towards the lower front of the neck, then select the thyroid gland. You can hide or fade other selected structures to adjust the view, or isolate the gland to work with it separately.
In the 3D model beside this article, rotate again or switch layers to change the view. You can also cut a section along the sagittal, coronal or transverse plane, or pull parts apart before rotating and zooming again.