Involution of an organ is the shrinking or return to a former size.
The thymus continues to grow between birth and puberty and then begins to atrophy, a
process directed by the high levels of circulating sex
hormones. Proportional to thymic size, thymic activity (T cell output) 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
results in the thymus increasing in size and activity.
It is a physiological process occurring after
the hypertrophy of the uterus has to be undone since it does
not need to feed the fetus anymore.
During pregnancy until after birth, mammary
glands grow steadily to a size
required for optimal milk production. At the end of breastfeeding,
the number of cells in the mammary gland becomes reduced until approximately
the same number is reached as before the start of pregnancy.