The thymus is a thin, bilobed organ located in the superior mediastinum that has a variable size and shape. The thymus lies anteriorly in relationship to the heart and great vessels. The relative size of the thymus increases with expiration and decreases with inspiration. The thymus decreases in size during periods of stress, such as during sepsis. Occasionally the thymus may extend inferiorly to the level of the diaphragm.
Thymic contour is variable.
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