Thymus
The thymus is the small organ behind the breastbone where new T-cells are trained, and its decline is the single most consequential event in immune aging. It does not switch off in adulthood, though: it keeps turning out some new T-cells well into late adulthood, how much working thymus is left varies a lot from person to person, and in observational studies adults who lost their thymus died sooner, while heavy smokers with a healthier-looking thymus lived longer. Nothing has been shown to restore it.
There is no proven way to regrow or protect your thymus, so don't pay for "thymus regeneration" protocols. The habits that go with a healthier-looking thymus, not smoking, avoiding obesity and staying active, are worth doing for their own sake, but that link is observational.
What it does, and how it ages
T-cells are the white blood cells that recognise specific threats, such as a virus they have met before or a cell turning cancerous. They mature in the thymus, which sends a steady supply of fresh, "naive" T-cells out into the body, each able to recognise something new.
The organ keeps roughly the same overall size through life, but its working tissue starts dwindling from the first year of life. From the teenage years on, fat steadily fills the space around it while the working tissue keeps shrinking for the rest of life.[1] The output of new naive T-cells falls but does not stop, and measurable output continues well into late adulthood.[2] A 2026 study that read the thymus from routine chest scans in more than 27,000 adults found that how much healthy thymus remains declines with age but varies widely from person to person.[3]
As fewer new cells arrive, the immune system leans more on its existing memory cells. It becomes slower to recognise threats it hasn't seen before, which is part of why vaccines work less well and new infections hit harder with age.[4] The wider picture of immune aging is on Immunosenescence.
Does the adult thymus still matter?
Two recent observational lines suggest it does.
Removing it. Some adults have the thymus taken out during heart or chest surgery. Over the next five years they had more deaths and more cancers than similar surgical patients who kept it: about 8% versus 3% died.[5]
Reading it on a scan. In the chest-scan study, older heavy smokers whose thymus looked healthier were about half as likely to die over 12 years, and had less lung cancer and fewer deaths from heart and blood-vessel disease. In a smaller general-population group, only the heart and blood-vessel link held up after adjustment. A healthier-looking thymus also went with not smoking, less obesity and more physical activity.[6]
Neither study shows that anything you do to the thymus changes outcomes. They show that the organ still matters in adults, not how to keep it.
Exercise and the thymus
The best human evidence is a snapshot. People aged 55 to 79 who had cycled seriously for most of their adult lives had more fresh T-cells leaving the thymus than inactive people their age, at levels similar to young adults. Yet they carried just as many worn-out T-cells that have lost CD28, a molecule T-cells need for proper activation.[7] Because people who stay this active may differ in other ways, it cannot show that the cycling preserved the thymus. No trial has shown that exercise protects it.
"Thymus regeneration"
An often-cited regimen of growth hormone, DHEA (dehydroepiandrosterone, a hormone supplement) and metformin rests on one uncontrolled pilot in 9 men. The trial was run by a company that holds a related patent, and the thymus looked better on scans in 7 of the 9.[8] Its controlled follow-up passed its expected end date in late 2025 with no results posted.[9] Treat it as unproven.
Can you measure your own?
Not in any way you can act on. The thymic-health score read from chest scans is one of several research tools, alongside immune-age scores built from blood, that are linked to mortality in large groups of people. None can yet tell you whether a change you make is working.[10][11]
What to do
- Don't buy thymus-regeneration protocols. The evidence is one small uncontrolled pilot.
- Do the ordinary things linked to a healthier-looking thymus: don't smoke, avoid obesity, stay active. The link is observational, and each is worth doing anyway.
- Because the aging immune system responds less to new threats, keep up the vaccines designed for older adults. See Immunosenescence.
Further reading
- Steinmann GG, Klaus B, Müller-Hermelink HK. The involution of the ageing human thymic epithelium is independent of puberty. A morphometric study. Scand J Immunol 1985.[12]
- Douek DC, et al. Changes in thymic function with age and during the treatment of HIV infection. Nature 1998.[13]
- Kooshesh KA, et al. Health Consequences of Thymus Removal in Adults. N Engl J Med 2023.[14]
- Bernatz S, et al. Thymic health consequences in adults. Nature 2026.[15]
- Duggal NA, et al. Major features of immunesenescence, including reduced thymic output, are ameliorated by high levels of physical activity in adulthood. Aging Cell 2018.[16]
- Fahy GM, et al. Reversal of epigenetic aging and immunosenescent trends in humans. Aging Cell 2019.[17]