Final answer:
The thymus involutes during puberty primarily due to the maturation of the immune system and is influenced by an increase in sex hormones like estrogen and testosterone. Hormonal shifts signal that a diverse T-cell repertoire has been established, thus reducing the need for a large thymus.
Step-by-step explanation:
The involution of the thymus during puberty is primarily due to the maturation of the immune system. As a person matures, the thymus, an organ behind the sternum and involved in the development and maturation of T-cells, begins to decrease in size. This process, known as thymic involution, is influenced by the rise of sex hormones such as estrogen and testosterone that enhance involution. The increase in these hormones during puberty signals the body that a sufficiently diverse T-cell repertoire has been established, reducing the need for a large thymus. Additionally, with age, there is a decrease in the expression of the FOXN1 gene, which plays a crucial role in maintaining thymic epithelial cells that support the thymus's function in T-cell maturation.