The thymus is a T-cell producing lymphoid organ in the anterior mediastinum that plays a role in the development of the immune system, particular the maturation of T-cells. It typically has a retrosternal location and hence can mimic retrosternal pathology.

It is relatively large in infancy (weighing 25 g at birth) reaching a maximal weight in adolescence between 12 and 19 years (35 g), and gradually involuting with age (between 20 and 60 years) with progressive fatty replacement of the cellular components (15 g at 60 years of age). Fatty replacement starts at puberty. There can be a wide variation in size between patients 3.

When sizeable, it consist of two lateral lobes touching in the midline, situated partly in the thorax, partly in the neck. The two lobes are usually asymmetric in size. They are occasionally united, so as to form a single mass. Sometimes they are separated by an intermediate lobe. 

The gland extends from as high as the lower border of the thyroid gland to the fourth costal cartilage downwards. 

  • parasternal, brachiocephalic and tracheobronchial lymph nodes
  • vagal fibres
  • sympathetic fibres entering with blood vessels that are vasomotor

The thymus is of a pinkish-gray colour, soft, and lobulated on its surfaces.  

  • 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

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 airway. The size can vary with inspiration.

  • typically uniformly soft tissue density, approximately 80 HU and isoattenuating to surrounding muscle
  • smooth outline with convex borders in childhood
  • triangular in adulthood
  • small blood vessels may be seen traversing it
  • typically demonstrates chemical shift artefact 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 >20 years of age
    • absence of a diagnosis associated with thymic enlargement or hyperplasia, e.g. Graves disease
  • 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

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

Embryologically it is derived from the 3rd pharyngeal pouch. 

"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 the plant 7.

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

Anatomy: Thoracic
Share article

Article information

rID: 7337
System: Chest
Section: Anatomy
Synonyms or Alternate Spellings:
  • Thymic tissue
  • Thymic gland

Support Radiopaedia and see fewer ads

Cases and figures

  • Drag
    Wavy thymus

Note...
    Case 1: wavy thymus
    Drag here to reorder.
  • Drag
    Thymus - right si...
    Case 2: right sided
    Drag here to reorder.
  • Drag
    Case 3: prominent thymus
    Drag here to reorder.
  • Drag
    Case 4: left sided
    Drag here to reorder.
  • Drag
    Case 5: normal thymus
    Drag here to reorder.
  • Drag
    Case 6: ectopic cervical position
    Drag here to reorder.
  • Drag
    Case 7: normal thymus on CT 12 year old
    Drag here to reorder.
  • Drag
    Case 8: thymic sail sign
    Drag here to reorder.
  • Updating… Please wait.
    Loadinganimation

    Alert accept

    Error Unable to process the form. Check for errors and try again.

    Alert accept Thank you for updating your details.