Thymic involution

Thymic involution is the slow replacement of your thymus tissue with fat. It starts in early childhood and speeds up at puberty. As it progresses, the thymus makes fewer fresh (naive) T cells from bone-marrow precursors. By your 60s, the output of diverse naive T cells has shrunk to a small fraction of the adolescent peak. That limits the variety of T cells available to fight new threats. Researchers have tried several ways to regrow the thymus. One is the TRIIM trial (Fahy and colleagues), where a growth hormone, DHEA, and metformin combo was linked to partial epigenetic age reversal. Another is giving interleukin-7 to support naive T cells in the body. But both are early-stage and need replication in larger trials before any clinical conclusions. Improving thymic function is seen as a plausible route to partly reversing age-related immune decline.

Last reviewed: June 22, 2026

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