Thymus: how this immune organ atrophies with age

The thymus is the gland tucked behind the breastbone where T lymphocytes are formed, and it starts to atrophy as early as the first year of life. This decline, called thymic involution, partly explains why vaccines protect less well and why infections take a heavier toll after age 65. Here is what studies actually measure: how fast the organ changes, what its fading means and which everyday levers seem to slow the process.

Thymus: definition and role in immunity

The thymus is a primary lymphoid organ located in the mediastinum, just above the heart and behind the breastbone. It receives precursor cells from the bone marrow and turns them into T lymphocytes, the cells that coordinate defense against viruses and spot cancer cells. Without it, a child is born with no functional T cell immunity: this is what is seen in DiGeorge syndrome, as immunologists at Duke point out (Haynes et al., 2000).

The organ is divided into two zones. In the cortex, young lymphocytes learn to recognize the body’s own molecules. In the medulla, those that would react against our own tissues are eliminated. Only a small minority of the cells that enter the organ ever leave it: this strict selection limits the risk of autoimmune disease. The cells that exit, called naive T lymphocytes, form a highly varied repertoire, able to respond to microbes never encountered before.

This repertoire is maintained throughout life, but it relies on two sources: thymic output and the division of lymphocytes already present. When the first weakens, the second takes over, at the cost of lower diversity. This shift is what links the gland to immune aging, described in our feature on immunity and age.

How the thymus atrophies with age: the mechanism of involution

Involution of the thymus does not start at puberty, contrary to a long-held belief: it begins in the first year. Steinmann’s team measured the thymic tissue of 204 people aged 1 month to 107 years. The volume of the epithelium, the tissue that produces lymphocytes, declines continuously along a logarithmic curve, and puberty changes neither its speed nor its nature (Steinmann et al., 1985).

The most surprising finding of this study concerns the size of the organ: it stays stable, around a median of 19.5 cm3. So the organ does not really shrink; it changes composition. Fat moves in massively after age 15 and takes the place of the active zones. The main documented changes are the following:

• the thymic epithelium, cortex and medulla, loses volume every year;
• the perivascular space, which plays no role in making lymphocytes, widens until age 10 to 25 and then fills with adipose tissue;
• the supporting cells of the niche, which attract precursors coming from the bone marrow, become scarcer;
• the output of naive T lymphocytes falls, and the repertoire becomes less diverse.

Several factors sustain this atrophy. Thymic epithelial cells age, growth signals such as interleukin 7 decrease, and interleukin 6, a pro-inflammatory molecule, promotes thymic atrophy. Palmer’s review stresses that these mechanisms remain poorly understood and combine causes internal to the organ with signals from the rest of the body (Palmer, 2013).

What science says about the aging thymus

The adult thymus remains active, far more than was believed for decades. In 1998, Douek’s team quantified thymic output using TRECs, DNA fragments left behind by each newly made T lymphocyte. Output declines with age, but substantial production persists into late adulthood (Douek et al., 1998). In 87 normal organs collected from birth to age 78, the Duke team confirmed that lymphocyte production continues normally in the remaining epithelial zones (Flores et al., 1999).

The most striking study comes from Massachusetts General Hospital. Kooshesh and colleagues compared 1,146 adults whose thymus had been removed during cardiothoracic surgery with matched patients who had surgery without thymectomy. At 5 years, all-cause mortality reached 8.1% versus 2.8% (relative risk 2.9), and cancer risk 7.4% versus 3.7% (Kooshesh et al., 2023). In a subgroup followed for 14 years on average, markers of new CD4 T cell production were almost three times lower after removal, with more inflammatory cytokines in the blood. The study is observational: it shows a strong association, not proven causation.

In children, removing the gland during heart surgery also leaves a mark. A research team at the Pitié-Salpêtrière hospital in Paris observed a loss of naive lymphocytes in young adults who had undergone thymectomy in early childhood. A subgroup of 22-year-olds showed an immune profile usually seen after age 75, linked to cytomegalovirus infection (Sauce et al., 2009).

Atrophy of the thymus also contributes to inflammaging, the chronic low-grade inflammation linked to age described by Franceschi and Campisi (2014). Self-reactive lymphocytes poorly eliminated by an aging organ and the reduced clearance of senescent cells keep this inflammatory background noise going (Thomas et al., 2020). Our levers against this phenomenon are detailed in the article on chronic inflammation.

In practice: what speeds up or slows thymus atrophy

Excess body fat is the best documented accelerating factor. In mice, diet-induced obesity causes premature thymic involution and reduces T cell diversity. In middle-aged humans, progressive adiposity, with or without type 2 diabetes, is associated with lower thymic output (Yang et al., 2009).

Conversely, two accessible levers have been associated with better thymic function. Physical activity first: 125 amateur cyclists aged 55 to 79, active for decades, produced as many recently made T lymphocytes as young adults, with more protective interleukin 7 and less interleukin 6 (Duggal et al., 2018). The study is cross-sectional and does not prove causation. Moderate calorie restriction second: in the CALERIE trial, a reduction of about 14% in calories over 2 years improved lymphocyte production and reduced the fat lodged in the organ (Spadaro et al., 2022).

Attempts at pharmacological regeneration remain experimental. The TRIIM trial combined growth hormone, DHEA and metformin in 9 men: it reports signs of thymic regeneration and an epigenetic age 1.5 years below baseline after one year (Fahy et al., 2019). With no control group, a tiny sample and authors who held shares in the sponsoring company, this result justifies no self-medication. On the research side, thymic niche cells grafted into mice restarted lymphocyte production and the vaccine response (Gustafsson et al., 2025). MedlinePlus, from the US National Library of Medicine, describes the broader aging changes in the immune system.

This drop in thymic output has a concrete consequence: vaccines trigger a weaker response with age, which justifies the enhanced formulations presented in our article on vaccination and age. The thymus belongs to the pillar altered intercellular communication, one of the hallmarks of aging.

Tips to preserve your thymus function

No supplement or food has been shown to regrow a thymus in humans. The steps below target the factors associated with faster atrophy in studies, without promising regeneration:

• Move every week, with endurance exercise sustained over time: the cyclists studied by Duggal had been riding for decades.
• Watch your waistline, as visceral fat goes hand in hand with lower thymic output.
• Avoid chronic calorie excess; severe restriction without medical supervision is not recommended, especially after 70 because of the risk of muscle loss.
• Keep your vaccinations up to date after 65, since the repertoire of new lymphocytes is shrinking.
• Tell your surgeon about any history of thymectomy and have it recorded in your medical file.

These measures also act on low-grade inflammation, muscle mass and cardiovascular risk, which makes them safe choices even if their specific effect on the gland remains modest. To receive our summaries on immunity and healthy aging, subscribe to our health newsletter.

Frequently asked questions about the thymus

At what age does the thymus start to atrophy?

The thymus starts to atrophy in the first year of life. A morphometric study of 204 people aged 1 month to 107 years showed that its lymphocyte-producing tissue declines continuously, with no acceleration at puberty. Fat massively replaces the active zones after age 15, while the total volume of the organ stays roughly stable.

Does the thymus disappear completely in adults?

No, the thymus does not disappear in adults. It fills with fat, but islands of active tissue persist into old age. TREC measurements, markers of newly made T lymphocytes, show lower but still present production after 60. An analysis of 87 thymus glands collected up to age 78 confirms this.

Can you live without a thymus?

Yes, you can live without a thymus, but with measurable consequences. In adults who had heart or chest surgery, removal was associated with a 5-year mortality of 8.1% versus 2.8% and a doubled cancer risk. These figures come from a 2023 observational study: they show an association, without proving causation.

How can you slow thymus aging?

Human data link better thymus function to sustained physical activity and the absence of excess fat. Cyclists aged 55 to 79 produced as many new lymphocytes as young adults. A 14% calorie restriction over 2 years also improved this production. No drug or supplement has been validated to regenerate the organ.

What is the difference between the thymus and the thyroid?

The thymus and the thyroid are two distinct organs. The thyroid, at the base of the neck, produces hormones that regulate metabolism. The thymus, located lower behind the breastbone, forms the T lymphocytes of the immune system. Their proximity and similar names feed the confusion, but their functions and diseases are unrelated.