Thymus Gland

Reading Time:


The thymus is located in the neck and thorax. It is a pink lobulated organ involved in the development of the immune system before puberty, after which it atrophies and is replace with fatty tissue.

Gross Anatomy

The thymus is an asymmetrical, bilobed lymphoid organ. The gland mainly lies in the superior mediastinum, but the upper portion can extend to the thyroid in the neck and the lower portion typically lies in the anterior mediastinum, posterior to the manubrium and anterior to the fibrous pericardium.


In the embryo the thymus is derived from the third pharyngeal pouch and relative to body size the thymus is largest in the infant. During adolescence the thymus is involved in the immune systems' early development; after puberty the thymus atrophies and the tissue is replaced with fat.


The lobules of the thymus are made up of follicles, which consist of a central medulla surrounded by a peripheral cortex. The cortex contains lymphocytes and epithelial reticular cells, the medulla has fewer lymphocytes but more epithelial reticular cells.


Branches of the internal thoracic and inferior thyroid arteries supply the thymus and venous drainage is mainly into the left brachiocephalic and some into the internal thoracic veins. There is minimal nervous supply to the thymus and no afferent lymphatics. Lymphatic drainage from the thymus is into parasternal, tracheobroncial and brachiocephalic nodes.

Clinical Anatomy

DiGeorge Syndrome - This is a genetic syndrome caused by deletions on chromosome 22. Clinical findings include thymic aplasia, congenital heart defects, abnormal facies, hypoparathyroidism and cleft palate. Individuals with an aplastic thymus have an underdeveloped immune system and are therefore susceptible to repeated infections.


Ectopic parathyroid glands - The thymus and parathyroid glands both develop embryologically from the third pharyngeal pouch resulting in the thymus as a common site from ectopic parathyroid glands and possible ectopic parathyroid hormone production.

Quick Anatomy

Key Facts

Developmental precursor

Arterial supply

Venous drainage

Lymphatic drainage

Third pharyngeal pouch.

Branches from the internal thoracic and inferior thyroid arteries.


Left brachiocephalic vein, internal thoracic veins.

Parasternal, tacheobroncial and brachiocephalic nodes.



The 5 T's - These are the most common causes of anterior mediastinal masses:

1 - Thymus, thymoma.

2 - Thyroid, ectopic thyroid masses.

3 - Thoracic aorta, dilated or anaeurysm of the ascending aorta.

4 - Terrible lymphoma.

5 - Teratoma and germ cell tumors.


The thymus is a lobulated lymphoid organ with an important role in immune development before puberty. It is located mainly in the superior mediastinum, posterior to the manubrium. Follicles make up the thymus tissue, which are arranged with a central medulla and a peripheral cortex consisting of lymphocytes and epithelial reticular cells.


Lorem ipsum dolor sit amet, sapien platea morbi dolor lacus nunc, nunc ullamcorper. Felis aliquet egestas vitae, nibh ante quis quis dolor sed mauris.