Thymus

Thymus is a bi-lobed, lymphoid gland, 3-4 cm diameter, in the anterior superior mediastinum
Thymus primarily functions as a host for T Cell maturation and differentiation
1. After forming in Bone Marrow, T Cells migrate to the Thymus for maturation
2. Within the Thymus, T Cells differentiate into two cell lines with different T-Cell Receptors (CD4 and CD8)
3. After maturation and differentiation, T Cells are released from the Thymus into the peripheral circulation
4. T-Cells are named for their Thymus origin
Thymus fully develops in utero, with maximal size in the first 2 years of life
1. Thymus involutes with Puberty, replaced by fatty infiltration
2. However, Thymus appearance on XRay is only evident for first 2 years of life (see below)
Anatomy
1. Lewis (1918) Gray's Anatomy 20th ed (in public domain at Yahoo or BartleBy)

Thymus appearance on Chest XRay
1. Triangular mass in the upper chest, anterior-superior mediastinum
Increases in size until age 12 months, then becomes much less evident by age 2 years
Typically larger on the right side of the upper chest (AP/PA)
Differentiate Thymus Shadow from Cardiomegaly in infants
1. See Congenital Heart Disease
2. See Pediatric Congestive Heart Failure
3. Findings suggestive of cadiomegaly
  1. Boot-shaped heart (AP/PA)
  2. Posterior heart shadow edge overlap with Vertebral bodies (lateral)
  3. Anterior tracheal line displaced by posterior heart border (lateral)

Differential Diagnosis
Lesions Similar to Thymus Shadow on XRay (children age >2 years, teens and adults)