Longevity science: Once considered useless in adults, this tiny organ can determine how long you live

For years, the thymus was one of those organs that did not get much attention after childhood. It sits behind the breastbone and plays an important role in training the immune system when we are young, but it shrinks considerably with age. Because of that, scientists had largely believed that its importance faded away in adulthood.

New research is now challenging that idea. A study published in Nature suggests that the condition of the thymus in adults may be closely linked to how long people live and their risk of developing serious diseases.

The small organ that changes as we age


The thymus is where T cells, an important part of the immune system, mature and learn to recognise harmful invaders while avoiding healthy cells. It reaches its largest size around adolescence and then gradually becomes smaller as it is replaced by fatty tissue.

For a long time, this process, known as thymic involution, was taken to mean that the organ had largely finished its job.

But the new study found that the amount of thymic function that remains in adults can vary quite a lot from one person to another.

Researchers analysed CT scans and health information from 27,612 people involved in the National Lung Screening Trial and the Framingham Heart Study. They developed a deep-learning system that examined the size and composition of the thymus and gave participants a thymic health score.

Healthier thymus linked to lower death risk

The results were striking. In the National Lung Screening Trial, people with higher thymic health had about a 50% lower risk of death during the 12-year follow-up period compared with those who had low thymic health.

They were also 36% less likely to develop lung cancer and nearly 50% less likely to die from lung cancer. The researchers also found a lower risk of cardiovascular death, with reductions ranging from 63% to 92% across the two study groups.

Hugo Aerts, an oncologist at Harvard University and director of the Artificial Intelligence in Medicine Program at Mass General Brigham, said the team had not expected the connection to be so strong.

"We suspected the adult thymus might still matter, but we were surprised," Aerts told ScienceAlert.

He added: "We did not expect these differences to be so strongly and consistently associated with longevity, cancer incidence, cardiovascular disease, and cancer therapy response across multiple large, independent cohorts. That suggested the adult thymus is far more important than previously appreciated."

It may also affect cancer treatment

The findings do not stop at longevity. A companion Nature study involving 3,476 cancer patients found that thymic health was also associated with outcomes among people receiving immune checkpoint inhibitor treatments. Patients with healthier thymuses showed better outcomes, particularly in studies involving non-small-cell lung cancer.

There is also earlier evidence that the adult thymus may matter. A 2023 study published in the New England Journal of Medicine found higher five-year mortality and cancer risk among adults who had undergone thymus removal compared with matched controls.

Still, researchers are careful about what the new findings mean. The Nature study shows an association, not proof that a healthier thymus directly causes people to live longer. Smoking, obesity and other factors were also linked with poorer thymic health.

Aerts said: "I'm excited because this is more than the discovery of a new biomarker."

He added: "It suggests that we've overlooked a major aspect of human biology for decades. If these findings continue to be confirmed, preserving and measuring immune health could become a new principle of medicine, much like we currently monitor heart or kidney function. The thymus may be one of the first organs that allows us to do that in a practical way."