The thymus is an organ located in the upper anterior portion of the chest cavity just behind the sternum. The thymus continues to grow between birth and puberty and then begins to atrophy. Proportional to thymic size, thymic activity is most active before puberty. Upon atrophy, the size and activity are dramatically reduced, and the organ is primarily replaced with fat. The atrophy is due to the increased circulating level of sex hormones, and chemical or physical castration of an adult result in the thymus increasing in size and activity.

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