The thymus (plural: thymi) is a lymphoid organ in the anterior mediastinum responsible for the production and maturation of T-cells until puberty. It is a vital component of the immune system and plays a role in prevention of cancer ¹², infection and autoimmunity ¹¹.

Gross anatomy

It is relatively large in infancy (weighing 25 g at birth) and grows considerably immediately after birth. Maximum weight of around 35g is achieved at puberty after which time it undergoes involution with progressive fatty replacement, reaching a weight of around 15g at 60 years of age. Involution occurs more rapidly in males. There can be a wide variation in size between patients ³.

The two lobes are usually asymmetric in size, the left lobe being higher and larger than the right ¹⁰. The lobes are occasionally united or may be separated by an intermediate lobe. Depending on age and size, the normal gland may extend from the lower border of the thyroid gland to the fourth costal cartilage. 

Relations

• anteriorly: sternum, origins of the inferior strap muscles

• posteriorly: pericardium, aortic arch, great vessels, left brachiocephalic vein, trachea

• laterally: pleura, pretracheal fascia

Arterial supply

• inferior thyroid and internal thoracic arteries

Venous drainage

• left brachiocephalic, internal thoracic and inferior thyroid veins

Lymphatic drainage

• parasternal, brachiocephalic and tracheobronchial lymph nodes

Innervation

• vagal fibers

• sympathetic fibers entering with blood vessels that are vasomotor

Histology

The thymus is of a pinkish-grey color, soft, and lobulated on its surfaces.  

Development

Embryologically it is derived from the third pharyngeal pouch. The thymus is the first of the lymphoid organs to be formed. Considerable growth occurs immediately after birth in response to antigen stimulation and demand for mature T cells. Genetic factors also influence dependence upon thymus immunological function. After fibrofatty atrophy, the thymus can grow back at any time in life, particularly after periods of stress.

Variant anatomy

• variable location: ectopic and/or accessory thymic tissue may be located anywhere along the path of descent of the thymopharyngeal ducts, e.g. retrocaval, cervical, posterior mediastinal

• variable shape: e.g. unilobed, trilobed, X-shaped, inverted V-shaped, etc.

Radiographic features

Plain radiograph

The thymus can be seen on chest radiographs within 24 hours after birth, then becomes smaller after the age of 2 years. It is rarely seen after the age of 8 years ¹⁰.

The thymus is seen as a triangular sail (thymic sail sign) frequently towards the right of the mediastinum. It has no mass effect on vascular structures or airways. The size can vary with inspiration.

Ultrasound

• typically relatively homogeneous background echogenicity similar to or slightly less than that of the liver and spleen, with scattered hyperechoic foci resembling a starry sky

• its shape can be affected/distorted by cardiac pulsations and respiratory motion as it is soft and pliable and should not compress or displace adjacent structures

CT

• typically uniformly soft tissue density, approximately 80 HU and isoattenuating to surrounding muscle

• smooth outline with convex borders in childhood

• triangular in adulthood, usually measures less than 1 cm in diameter ¹⁰

• small blood vessels may be seen traversing it

MRI

• typically demonstrates chemical shift artifact between in and out of phase images

• differentiating normal from hyperplastic thymus can be difficult and guidelines for making this distinction and verifying the presence of normal thymus include ⁵:

  • absence of rounded soft-tissue masses >7 mm

  • absence of a convex contour of the thymus >19 years of age

  • absence of soft-tissue lobulation

  • absence of excessive thymic thickness (should be ≤1.3 cm when >20 years of age)

  • absence of a diagnosis associated with thymic enlargement or hyperplasia, e.g. myasthenia gravis 

History and etymology

"Thymus" ultimately derives from the Greek word for the plant "thyme" θύμος ("to offer/sacrifice"), presumably because the plant was burnt on altars. Galen thought the thymus gland looked like a "warty excrescence" and resembled a bundle of plants ⁷.

The first good description of the thymus gland was recorded by Berengarius in 1524.

Related pathology

• thymic rebound

• thymic epithelial tumors

• primary tumors of the thymus

• thymic hyperplasia

• thymic amyloidosis

• DiGeorge syndrome

• the normal thymus can mimic several pathologies:

  • retrosternal hematoma

  • spinnaker sign in pneumomediastinum

  • aortic injury