Because of its relative prominence in infancy, a normal thymus gland can give the impression of cardiomegaly, a mediastinal mass or even lung pathology. Recognition of certain radiographic characteristics that pertain to this normal structure help to differentiate it from pathology.

THE THYMUS

The thymus is a soft organ comprising two asymmetric lobes joined by an isthmus and invested by a fibrous capsule. Embryologically it is derived from the third pharyngeal pouch. The thymus occupies the anterior and superior mediastinum, located behind the sternum in the midline. In infancy the upper border of the thymus extends almost to the thyroid gland in the neck and the lower margin overlaps the upper part of the heart, occasionally extending as far as the diaphragm. It can be thought of as a soft pancake-like structure draped over the mediastinal vessels.

The thymus has a crucial role in the development of immunity, although its function has only been elucidated fairly recently. It serves as the site of differentiation of T lymphocytes in all mammals. These migrate to the thymus during fetal life and early infancy, emerging after a process of molecular modifications as specific types of immunocompetent cells involved in cell mediated immunity. Mature immunocompetent cells emerging from the bone marrow or thymus circulate and populate secondary lymphoid organs—the spleen and lymph nodes as well as aggregates of lymphatic tissue associated with mucosal surfaces such as the gut and bronchial system. The critical role of the thymus is completed in the first few days of life but it does have a continued role in immunity throughout life.

The thymus has its largest size relative to body weight at birth, weighing on average 10–15 g. It has an extremely variable size and appearance during early life but grows to a maximum weight of approx 35 g at …