Thymus: Location, Relations, Development and Clinical Anatomy

By Dr Richard Miller, MBChB FRCS · Reviewed 1 October 2026

The thymus is a bilobed primary lymphoid organ in the superior and anterior mediastinum, lying behind the manubrium and in front of the great vessels and pericardium. It is where T lymphocytes mature. It is largest relative to body size in early childhood, then involutes after puberty and is largely replaced by fat in adults.

Thymus · key facts

Location: Superior and anterior mediastinum, behind the manubrium and upper sternal body; may extend into the lower neck
Blood supply: Branches of the internal thoracic and inferior thyroid arteries
Venous drainage: Thymic veins to the left brachiocephalic vein, and to internal thoracic and inferior thyroid veins
Lymphatic drainage: Efferent vessels only, to parasternal, brachiocephalic and tracheobronchial nodes
Nerve supply: Sympathetic fibres from the cervicothoracic ganglia and branches of the vagus
Function: Maturation and selection of T lymphocytes

Structure

The thymus consists of two asymmetrical lobes joined by connective tissue, each divided into lobules with an outer cortex and inner medulla.

In a child the thymus is a soft, pinkish-grey, lobulated organ, often extending up into the neck towards the thyroid gland and down over the pericardium. After puberty the lymphoid tissue steadily gives way to fat. In an adult cadaver it is usually a yellow fatty mass in front of the great vessels, recognisable by its position and faint lobulation.

A thin capsule sends septa inwards to divide each lobe into lobules. The cortex is densely packed with developing T cells (thymocytes) supported by epithelial reticular cells, and has a blood and thymus barrier that shields maturing cells from circulating antigen. The medulla is paler, with fewer lymphocytes and the characteristic thymic corpuscles (Hassall's corpuscles), whorls of epithelial cells.

Development

The thymus develops from the endoderm of the third pharyngeal pouch on each side, along with the inferior parathyroid glands. The two primordia descend into the thorax and meet in the midline, dragging the inferior parathyroids down with them. This shared origin explains ectopic thymic tissue in the neck, and why an inferior parathyroid can lie in or near the thymus.

Relations

The thymus sits between the sternum in front and the great vessels and pericardium behind, with the pleura and phrenic nerves on either side.

Direction	Relation
Anterior	Manubrium and upper body of sternum; origins of sternohyoid and sternothyroid; internal thoracic vessels anterolaterally
Posterior	Left brachiocephalic vein, arch of the aorta and its branches, pericardium; trachea above
Lateral	Mediastinal pleura and lungs; phrenic nerves
Superior	Lower neck, towards the thyroid gland, in children

Blood supply and innervation

The thymus is supplied mainly by small branches of the internal thoracic arteries, with contributions from the inferior thyroid arteries.

The veins drain chiefly into the left brachiocephalic vein, which lies directly behind the gland, and also into the internal thoracic and inferior thyroid veins. These veins are short and can tear during thymectomy. The thymus has no afferent lymphatic vessels, unlike a lymph node; efferent vessels pass to the parasternal, brachiocephalic and tracheobronchial nodes. Nerves come from the sympathetic trunk via the cervicothoracic ganglia and from the vagus.

Clinical relevance

The thymus matters clinically as a cause of anterior mediastinal masses and for its link with myasthenia gravis.

Myasthenia gravis: many patients have thymic hyperplasia, and a minority have a thymoma. Thymectomy improves the disease in selected patients.
Thymoma: the commonest primary tumour of the anterior mediastinum in adults.
Thymectomy: performed through a median sternotomy or by video-assisted or robotic surgery. The phrenic nerves on each side mark the lateral limits of resection and must be preserved, and the left brachiocephalic vein must be protected.
DiGeorge syndrome (22q11.2 deletion): failure of third and fourth pouch development causes thymic hypoplasia with T-cell deficiency, together with hypocalcaemia from absent parathyroids and cardiac outflow defects.
Paediatric radiographs: the normal large infant thymus can widen the mediastinum and produce the triangular sail sign, which should not be mistaken for disease.

How it is examined

On a cadaveric spotter, in an OSPE and in MRCS Part B anatomy.

On a thoracic prosection, the fatty tissue behind the manubrium and in front of the left brachiocephalic vein is the thymic remnant; the vein directly behind it is the most common structure pinned alongside.
The classic viva question is the embryology: third pharyngeal pouch with the inferior parathyroids, and why an inferior parathyroid adenoma may be found in the mediastinum.
Expect the differential of an anterior mediastinal mass and the association of thymoma with myasthenia gravis.
Know which nerves are at risk in thymectomy: the phrenic nerves on the pericardium on each side.

Key points

The thymus is a primary lymphoid organ in the superior and anterior mediastinum.
It lies behind the manubrium and in front of the left brachiocephalic vein, aortic arch and pericardium.
It develops from the third pharyngeal pouch with the inferior parathyroid glands.
It involutes after puberty and is largely fat in adults.
Its veins drain mainly to the left brachiocephalic vein; it has no afferent lymphatics.
Thymoma and thymic hyperplasia are associated with myasthenia gravis.

Common questions

Where is the thymus located?

The thymus lies in the upper chest, in the superior and anterior mediastinum. It sits behind the manubrium and upper body of the sternum and in front of the left brachiocephalic vein, the arch of the aorta and the upper pericardium. In children it can reach up into the lower neck. The phrenic nerves and mediastinal pleura lie on either side.

What does the thymus do?

The thymus is where T lymphocytes mature. Immature cells arrive from the bone marrow and pass through the cortex and medulla, where cells that cannot recognise self MHC are removed and cells that react strongly against the body's own antigens are deleted. The surviving T cells leave to populate lymph nodes and the spleen. This activity is greatest in childhood.

Does the thymus disappear in adults?

The thymus does not disappear, but it shrinks and changes. After puberty its lymphoid tissue is progressively replaced by fat, a process called involution. In most adults it appears as a fatty mass with small islands of thymic tissue in front of the great vessels. It continues to produce some T cells, at a much lower rate than in childhood.

What is the link between the thymus and myasthenia gravis?

Myasthenia gravis is an autoimmune disease in which antibodies block acetylcholine receptors at the neuromuscular junction. The thymus is thought to be where this autoimmunity starts: many patients have thymic hyperplasia, and a minority have a thymoma. Removing the thymus, by sternotomy or minimally invasive surgery, improves symptoms and reduces medication needs in selected patients, and is the standard treatment for a thymoma.

References

Gray's Anatomy: The Anatomical Basis of Clinical Practice. Standring S (ed). Elsevier. 42nd edition, 2020.
Moore's Clinically Oriented Anatomy. Moore KL, Dalley AF, Agur AMR. Wolters Kluwer. 9th edition, 2022.
Gray's Anatomy for Students. Drake RL, Vogl AW, Mitchell AWM. Elsevier.