Thymus

The thymus is a T-cell producing lymphoid organ in the anterior mediastinum that plays a role in the development of the immune system. 

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 involutes with age (between 20 and 60 years) with progressive fatty replacement (15 g at 60 years of age). 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. 

Relations
Blood supply
  • arterial supply from the inferior thyroid and internal thoracic arteries
  • venous drainage from the thymus end of the left brachiocephalic, internal thoracic and inferior thyroid veins
Lymphatic drainage
  • parasternal, brachiocephalic and tracheobronchial lymph nodes
Innervation
  • vagal fibres
  • sympathetic fibres entering with blood vessels that are vasomotor

The thymus is of a pinkish-gray color, 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. uni-lobed, tri-lobed, X-shaped, inverted V-shaped
Plain radiograph

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.

CT
  • typically uniformly isoattenuating to surrounding muscle
  • smooth outline
MRI
  • typically demonstrates chemical shift artefact between in and out of phased 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 beyond  19 years of age
    • absence of soft-tissue lobulation
    • absence of excessive thymic thickness (should be ≤1.3 cm beyond age 20 years
    • absence of a diagnosis associated with thymic enlargement or hyperplasia, e.g. Graves disease

Embryologically it derives 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.


Thoracic anatomy
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Article Information

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

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    Wavy thymus

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    Case 1: wavy thymus
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    Thymus - right si...
    Case 2: right sided
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    Prominent thymus
    Case 3: prominent thymus
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    Case 4: left sided
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    Case 5: normal thymus
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    Case 6: ectopic cervical position
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    Case 6: on CT 12 y.o. boy
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