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 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 immuno-competent cells involved in cell mediated immunity.

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 puberty. Post-puberty the thymus involutes/atrophies, its glandular tissue gradually being replaced by fat.

The soft, normal thymus does not have any mass effect on adjacent structures such as the trachea, which should not be displaced or narrowed.

The antero-lateral margins of the gland are often indented by the anterior rib ends producing a scalloped appearance—the ‘‘wave’’ or ‘‘ripple’’ sign which may be seen on the left or right side