Anatomy Atlas

Thymus

Thymus
OrganHigh-yielddraftHeme

Primary lymphoid organ in the anterior and superior mediastinum where T cells mature. It is large in children, involutes after puberty, and thymoma is associated with myasthenia gravis.

Core anatomy

Location

Superior and anterior mediastinum, behind the manubrium and in front of the great vessels and pericardium. It may extend into the neck.

Parts
  • Two lobes, each with an outer cortex (immature T cells) and inner medulla (mature T cells, Hassall corpuscles)
  • Relatively large in infancy; replaced progressively by fat after puberty (involution)
Relations

Posterior to the sternum (manubrium); anterior to the left brachiocephalic vein, the arch of the aorta and its branches, and the pericardium.

Arterial supply

Branches of the internal thoracic artery and inferior thyroid arteries.

Venous drainage

Left brachiocephalic, internal thoracic and inferior thyroid veins.

Notes

Develops from the third pharyngeal pouch (endoderm), together with the inferior parathyroid glands.

Found in

Mentioned in

Generated automatically from links in other entries.

Divisions of the mediastinumA sagittal view of the chest with the sternum on the left and the thoracic vertebrae on the right. A plane from the sternal angle to the T4/T5 intervertebral disc separates the superior mediastinum above from the inferior mediastinum below. The pericardium and heart form the middle mediastinum; the narrow space between the sternum and the pericardium is the anterior mediastinum, and the space between the pericardium and the vertebrae is the posterior mediastinum. The superior mediastinum contains the thymus, great veins, aortic arch, trachea and esophagus; the posterior contains the descending aorta, esophagus, thoracic duct, azygos veins and sympathetic trunks. Anterior masses are the 4 T's, middle masses are lymph nodes and cysts, and posterior masses are neurogenic tumors.SuperiorAnt.MiddlePosteriorSternal angleT4/T5 discThymusTracheaEsophagusAortic archHeartDiaphragmThoracic inletAnterior ← → posterior · sagittal viewSuperiorThymus, great veins, aortic arch and branches,trachea, esophagus, thoracic duct, vagus andphrenic nerves, left recurrent laryngeal nerveAnteriorThymic remnants, fat, lymph nodesMasses: the 4 T’s — thymoma, teratoma,thyroid, “terrible” lymphomaMiddleHeart and pericardium, ascending aorta,pulmonary trunk, lower SVC, main bronchiMasses: lymph nodes, cystsPosteriorDescending aorta, esophagus, thoracic duct,azygos veins, vagus nerves, sympathetic trunksMasses: neurogenic tumors

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Divisions of the mediastinum. Sagittal view, anterior on your left. The plane through the sternal angle and the T4/T5 disc divides the superior from the inferior mediastinum; the pericardium splits the inferior part into anterior, middle and posterior. Masses follow the compartments: anterior = the 4 T's, middle = lymph nodes and cysts, posterior = neurogenic tumors. Radiologists increasingly use a three-compartment CT model (prevascular, visceral, paravertebral) that maps onto these.
Original schematic, created for this atlas.
Thymus exposed in the superior and anterior mediastinum, lying on the pericardium in front of the great vessels
The thymus in the anterior mediastinum of a full-term fetus.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 1178. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))

Clinical correlations

Thymoma and myasthenia gravis PANCE

Mechanism / lesion

Thymic epithelial tumor. A substantial proportion of thymoma patients have myasthenia gravis, and a minority of myasthenia patients have a thymoma; thymic hyperplasia is more common in MG. Other associations: pure red cell aplasia, hypogammaglobulinemia. verify

Classic presentation

Anterior mediastinal mass (often incidental), or symptoms of MG: fatigable weakness, ptosis, diplopia, bulbar symptoms. All MG patients should have chest CT to look for thymoma. Thymectomy benefits selected patients.

DiGeorge syndrome (22q11.2 deletion) PANCE

Mechanism / lesion

Failure of development of the 3rd and 4th pharyngeal pouches.

Classic presentation

Thymic aplasia (T-cell deficiency, recurrent viral/fungal infections), hypocalcemia (absent parathyroids, tetany), conotruncal heart defects (tetralogy of Fallot, truncus arteriosus), and facial anomalies. The thymic shadow is absent on neonatal CXR.

High-yield PANCE pearls

  • Anterior mediastinal mass: 4 T's: Thymoma, Teratoma (germ cell), Thyroid (retrosternal goiter), Terrible lymphoma.
  • Myasthenia gravis → CT chest for thymoma.
  • DiGeorge: absent thymus (no thymic shadow), hypocalcemia, conotruncal heart defects; 22q11.2 deletion.
  • The thymus is normally prominent on infant CXR (sail sign), not a mass.

Imaging anatomy

X-ray

In infants the normal thymus can look like a mediastinal mass; the "sail sign" (a triangular thymic shadow) is normal. The thymic shadow is absent in DiGeorge syndrome. In adults, anterior mediastinal masses are seen on the lateral film as retrosternal opacities.

CT

CT chest is the main test for thymoma (a well-defined anterior mediastinal mass) and for distinguishing it from lymphoma, germ cell tumors and goiter.

MRI

Chemical-shift MRI helps distinguish thymic hyperplasia (which contains fat and loses signal) from thymoma.

Key views & landmarks
  • CT chest with contrast (anterior mediastinum)

Physical exam

Strength / function testing

For MG: sustained upgaze (fatigable ptosis), the ice pack test (ptosis improves with cold), and repetitive testing of proximal muscle strength.

Self-quiz

Q1.

A 45-year-old woman has double vision and drooping eyelids that worsen at the end of the day. Acetylcholine receptor antibodies are positive. Which imaging study is indicated?

Show answer

B

Every patient with myasthenia gravis needs chest CT (or MRI) to look for a thymoma in the anterior mediastinum. Thymectomy is indicated for thymoma and can benefit selected patients without one.

Q2.

A newborn has tetany, recurrent thrush, and a conotruncal heart defect. The chest radiograph shows no thymic shadow. Which embryologic structures failed to develop?

Show answer

B

DiGeorge syndrome (22q11.2 deletion): the 3rd and 4th pharyngeal pouches fail to develop, so the thymus (T cells) and parathyroids (calcium) are absent, with conotruncal defects.

Q3.

Which is NOT one of the classic causes of an anterior mediastinal mass?

Show answer

E

Neurogenic tumors arise from the sympathetic chain and nerve roots in the posterior mediastinum. The anterior "4 T's" are thymoma, teratoma, thyroid, and "terrible" lymphoma.

Q4.

The thymus develops from which embryologic structure?

Show answer

B

The thymus develops from the third pharyngeal pouch (endoderm), together with the inferior parathyroid glands. DiGeorge syndrome reflects failure of the 3rd and 4th pouches.

Q5.

A chest radiograph of a healthy 6-month-old shows a triangular soft-tissue density extending from the upper mediastinum. What is it most likely to be?

Show answer

E

The thymus is relatively large in infancy and normally visible on CXR as the "sail sign"; it isn't a mass. It involutes after puberty and is replaced by fat.

Flashcards

See also