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Why does the thymus shrink during adolescence?

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atrophy One example of atrophy is the progressive loss of bone that occurs in osteoporosis (normal bone shown on left; osteoporotic bone shown on right).

The shrinking of the thymus during adolescence is a natural biological process known as involution, rather than a pathological condition of atrophy. While the term atrophy often refers to the wasting away of tissue due to disease or disuse, the thymus undergoes a programmed, gradual diminution in size starting at the onset of puberty.1

During this process, the thymic cortex becomes thin as lymphocytes disappear and are replaced by fat tissue originating from the partitions between the organ's lobules. This decline in thymic tissue is believed to be the primary reason for the age-related decrease in the production of T cells.12

Despite this reduction, the process is never fully complete. The remaining bits of thymic tissue are generally considered sufficient to maintain the organ's essential function of facilitating the maturation of lymphocytes, which are critical for the immune system.1