thymus (anatomy)

Last reviewed 01/2018

In the adult the thymus is sited within the superior mediastinum. Often, due to its regression after puberty, the earlier bilobed structure of the thymus is impossible to distinguish and it may be replaced by irregular areas of pink or brown fibrofatty tissue. Where there are identifiable lobes, they are thin and project vertically downwards in close apposition.

Its relations are:

  • anteriorly, from superficial to deep: manubium and body of sternum, attachments and superior projection of sternohyoid and sternothyroid, pretracheal fascia
  • anterolateral: costal cartilages and medial edge of mediastinal pleura
  • laterally: mediastinal pleura, phrenic nerve
  • posteriorly, from superior to inferior: trachea, branches of arch of aorta, left brachiocephalic vein, arch of aorta merging with the fibrous pericardium

In children, the thymus may be sited over the area between the inferior margin of the thyroid gland within the neck to the level of the third costal cartilage within the anterior mediastinum.