Normal Thymus

  • CXR: gone by ~ 5 years old, findings that help diagnose a normal thymus include 1) thymic wave sign = undulating thymus contour caused by ribs making impression on soft thymus that appear like waves and 2) thymic sail sign = triangle-shaped inferior margin of a normal thymus seen more commonly on the right upper lobe than the left upper lobe
  • CT: gone by ~ 17 years old
  • US: use a a problem solving tool, thymus has starry sky appearance
  • Fluoroscopy: moves with inspiration / expiration – increases in size in expiration
  • Imaging: shrinks when stressed by infection or malignancy, rebounds when disease resolves / treatment is over, make sure enlarging thymus not due to recurrent tumor in oncology patients
  • DDX: upper lobe consolidation due to pneumonia or atelectasis
  • DDX: anterior mediastinal tumor
    — Thymus is soft and is displaced by surrounding structures
    — Anterior mediastinal tumor is hard and displaces surrounding structures
  • DDX: lymphadenopathy
    — Lymph nodes are hard + cause airway obstruction + airway shift and lymph nodes are more mass like + resemble lobes of thymus less
    — Thymus is soft + compresses + displaces nothing, moves with inspiration / expiration – increases in size in expiration

Cases of Normal Thymus

CXR of normal thymus with wave sign
CXR AP shows an anterior mediastinal mass with a wave-like appearance to the left mediastinal contour.
CXR of normal thymus with sail sign
CXR AP and lateral shows a prominent anterior mediastinal mass whose right inferior margin appears triangular and sail-like in appearance.
CXR of normal thymus on rotated CXR
CXR AP rotated to the right shows complete opacification of the right lung apex. Repeat CXR without rotation was unremarkable.
US and CT of cervical thymus
Transverse+sagittal US of the neck (above) shows the mass to have a starry sky appearance. Axial+coronal CT with contrast of the neck shows the mass to be inferior to the thyroid and contiguous with the thymus.