[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Anatomy](https://durgical.com/en/anatomy/) › [Chest](https://durgical.com/en/anatomy/region/chest/) › Thymus [Lymphatic system](https://durgical.com/en/anatomy/system/lymphatic-system/) · [Chest](https://durgical.com/en/anatomy/region/chest/) # Thymus The thymus is a gland behind the upper breastbone that trains white blood cells (T cells) in childhood, then shrinks and is replaced by fat. [See in 3D →](https://durgical.app/?from=durgipedia&lang=en#region=thorax&select=thymus&side=r) Opens in the Durgical app, no sign-up needed. ## What it is A soft gland in the upper front chest, behind the sternum; large in children, mostly replaced by fat in adults. ## What it does Trains white blood cells (T cells) to recognize the body's own tissues during childhood. ## Key points - The main occupant of the anterior mediastinum, the compartment where thymoma, teratoma, retrosternal thyroid and lymphoma present as masses - About a third to a half of people with a thymoma have myasthenia gravis; about 10–15% of people with myasthenia have a thymoma, and many of the rest have thymic hyperplasia - Large in infants, where it can look like a sail on a [chest](https://durgical.com/en/anatomy/region/chest/) X-ray and be mistaken for a mass or pneumonia ## Key facts **Where** Superior and anterior mediastinum, behind manubrium; may extend into lower neck **Blood supply** Branches of internal thoracic arteries (and inferior thyroid arteries) **Nerve** Vagus and sympathetic branches (mainly vasomotor) **Function** T-lymphocyte maturation; large in childhood, replaced by fat after puberty ## Relations **In front** [Manubrium](https://durgical.com/en/anatomy/manubrium-of-sternum/) and upper body of the [sternum](https://durgical.com/en/anatomy/sternum/), the [sternohyoid](https://durgical.com/en/anatomy/sternohyoid-muscle/) and [sternothyroid](https://durgical.com/en/anatomy/sternothyroid-muscle/) muscles and the upper [costal cartilages](https://durgical.com/en/anatomy/costal-cartilage/) **Behind** [Pericardium](https://durgical.com/en/anatomy/pericardium/), [ascending aorta](https://durgical.com/en/anatomy/ascending-aorta/) and [aortic arch](https://durgical.com/en/anatomy/aortic-arch/), the left [brachiocephalic vein](https://durgical.com/en/anatomy/brachiocephalic-vein/) and, higher up, the [trachea](https://durgical.com/en/anatomy/trachea/) **Lateral** Mediastinal [pleura](https://durgical.com/en/anatomy/pleura/) with the [phrenic nerves](https://durgical.com/en/anatomy/phrenic-nerve/) running along its sides **Above** Its upper poles may reach the lower neck in front of the trachea, toward the [thyroid gland](https://durgical.com/en/anatomy/thyroid-gland/) ## How to find it Not palpable. It lies behind the manubrium in the anterior mediastinum, in front of the great vessels and the upper pericardium. Surgeons reach it through a median sternotomy or by thoracoscopy, tracing its upper horns toward the thyroid and keeping clear of both phrenic nerves at its sides. ## On imaging In infants, a smooth soft-tissue shadow widening the upper mediastinum (sail sign), with wavy edges from the costal cartilages. In adults, a triangle of fat in front of the aortic arch on CT; a solid nodule there points to thymoma or another anterior mass, and MRI chemical-shift imaging helps tell hyperplasia from tumour. ## Common problems - Thymoma, a usually slow-growing tumour, sometimes linked to myasthenia gravis. ## How it is diagnosed CT or MRI. ## Treatment - Surgical removal (thymectomy). ## Clinical notes - Thymectomy improves myasthenia gravis in many patients, even without a thymoma - Removed through a median sternotomy or by thoracoscopic or robotic surgery; the phrenic nerves on both sides are the structures to protect - Rebound thymic hyperplasia after chemotherapy or severe illness can mimic a tumour on imaging - Thymoma is linked to other autoimmune syndromes, such as pure red cell aplasia and low antibody levels (Good syndrome) ## Anatomical variants - Extension into the neck toward the thyroid gland, seen on ultrasound in about a third of children - Ectopic thymic tissue along its path of descent, from the angle of the [mandible](https://durgical.com/en/anatomy/mandible/) to the mediastinum ## Development Arises from the endoderm of the 3rd pharyngeal pouches, together with the inferior [parathyroid glands](https://durgical.com/en/anatomy/parathyroid-glands/), and descends into the chest; in DiGeorge syndrome (22q11 deletion) it fails to develop, with T-cell deficiency and low calcium. ## Memory aid The 4 Ts of an anterior mediastinal mass: Thymoma, Teratoma, Thyroid and Terrible lymphoma. ## Related conditions - [Adenosine deaminase deficiency (ADA-SCID)](https://durgical.com/en/conditions/adenosine-deaminase-deficiency/) - [Pure red cell aplasia (incl. Diamond–Blackfan anaemia)](https://durgical.com/en/conditions/pure-red-cell-aplasia/) - [Myasthenia gravis](https://durgical.com/en/conditions/myasthenia-gravis/) - [DiGeorge syndrome (22q11.2 deletion syndrome)](https://durgical.com/en/conditions/digeorge-syndrome/) - [Mediastinal masses (thymoma and anterior mediastinal mass)](https://durgical.com/en/conditions/mediastinal-masses/) - [Tetralogy of Fallot](https://durgical.com/en/conditions/tetralogy-of-fallot/) ## Test yourself Think of the answer, then tap the question to see it. ## In which mediastinal compartment does the thymus lie, and which masses present there? The anterior mediastinum (behind the manubrium); thymoma, teratoma, retrosternal thyroid and lymphoma present there as masses. ## Which autoimmune disease is linked to thymoma? Myasthenia gravis: about a third to a half of people with a thymoma have it, and thymectomy improves it in many patients. ## Which nerves must be protected during thymectomy? Both phrenic nerves, which run along the sides of the thymus on the mediastinal pleura. ## From which embryonic structure does the thymus arise, and what happens in DiGeorge syndrome? From the 3rd pharyngeal pouches, with the inferior parathyroids; in DiGeorge syndrome (22q11 deletion) it fails to develop, giving T-cell deficiency and low calcium. ## What is the thymic sail sign on an infant's chest X-ray? The large normal infant thymus widening the upper mediastinum like a sail, which can be mistaken for a mass or pneumonia. [Add to my cards (5)](https://durgical.app/estudiar/?add=/en/anatomy/thymus/) Educational information, not medical advice. For any symptom, see a health professional. ## Its names **Official Latin (TA2)** thymus TA2 5152 **Official Spanish (DeCS)** Timo DeCS D013950 **English (TA2)** Thymus **Also called** thymus gland ## Related - [Brachiocephalic veins](https://durgical.com/en/anatomy/brachiocephalic-vein/) - [Internal thoracic artery](https://durgical.com/en/anatomy/internal-thoracic-artery/) Sources: Terminologia Anatomica (TA2); [DeCS](https://decs.bvsalud.org/es/ths/resource/?id=14338); [Anatomy, Head and Neck, Thymus](https://www.ncbi.nlm.nih.gov/books/NBK539748/); [21.1 Anatomy of the Lymphatic and Immune Systems](https://openstax.org/books/anatomy-and-physiology-2e/pages/21-1-anatomy-of-the-lymphatic-and-immune-systems) (OpenStax (Rice University), 2022) · adapted, [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/anatomy/thymus/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/thymus/