In 1996, cryobiologist Gregory Fahy walked into his doctor’s office and got a monthly supply of growth hormone. Your hope, reinforced by a single study in rats¹was that the injections would help him grow his thymus, a peculiar immune organ that atrophies and virtually disappears as people age. Regenerating it, Fahy thought, would help it live a longer, healthier life.

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The improvement was obvious, at least according to the magnetic resonance imaging (MRI), says Fahy. His functional thymic mass almost doubled². However, it was not so clear whether that made him feel younger. “I was only 46 at the time and more or less in peak health,” he says, although subsequent attempts to regrow the tissue left him feeling “energized and invincible.”

What began as an unregulated self-experiment has become a series of small clinical trials conducted by a biopharmaceutical company called Intervene Immune in Torrance, California, where Fahy is chief scientific officer.

The company is not alone. The past three years have seen an explosion of research on the thymus, spurred by reports in the literature that the health of the organ, once considered dispensable, is a likely indicator of a person’s overall health. Excitement intensified when a pair of studies published this year reported that declining thymus health correlates with increased risk of death.^3,4.

Investors are responding. In January, a Basel, Switzerland, biotech company called TECregen raised 10 million Swiss francs ($12.4 million) to develop therapies to regenerate the thymus, which could help slow aging and prevent cancer, according to the company’s website. It has not disclosed any drug candidates. Last October, Zag Bio, a thymus-focused biotech company in Cambridge, Massachusetts, launched with $80 million in funding. Pharmaceutical and venture capital companies have approached Fahy and other scientists and started their own research programs. “The current interest in this field is enormous,” says Marcel van den Brink, physician-in-chief and president of City of Hope, a cancer hospital and research center in Duarte, California, who has studied the thymus since 1996. There was a “huge perception that I was wasting my time,” he says. But now “people are reaching out to us” and taking the field seriously. (van den Brink is a scientific co-founder of a biotech company called Thymofox in Cambridge, Massachusetts.)

forgotten organ

For decades, people thought the thymus was vestigial. It has been “forgotten” and “disrespected,” says Georg Holländer, an immunologist at the University of Oxford, UK, who co-founded TECregen. In the 1920s, some researchers thought that the organ was involved in the production of eggshells in birds, but that it had limited functions in mammals. Decades later, Nobel Prize-winning biologist Peter Medawar referred to the thymus as “a minor evolutionary accident.”

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They were wrong. The importance of the organ had eluded scientists for so long, in part because mammals that have their thymus surgically removed appear to be perfectly healthy. It wasn’t until the 1960s, when immunologist Jacques Miller performed surgeries on newborn mice, that the organ’s function was discovered.⁵. Baby mice without thymus often succumbed to the infection and died. They had few immune cells in their lymph nodes.

Miller discovered that the thymus creates T cells, a type of cell he also identified around the same time and which is named after the organ. We now know that T cells are a crucial part of the immune system that attacks cancer cells and fights infections.

In humans, the thymus shrinks dramatically throughout life. After puberty, most of your cells become non-functional adipose tissue. T cell production slows down. At age 40, the thymus produces about a quarter of the T cells it did at age 8. At 65, he produces only 10%.⁶.

Focus on function

For this reason, many scientists assumed that fresh T cells were no longer necessary for older people. “People have the impression that it’s only important to make T cells when you’re young,” says Jennifer Cowan, an immunologist at University College London. “And it’s clear that’s not the case.”

Three discoveries changed that paradigm. First, a 2023 study found that people who had their thymus surgically removed were almost three times more likely to die and twice as likely to develop cancer five years after surgery than people who did not have their thymus removed.⁷. “It was very surprising,” Cowan says, “to the point that people won’t want to have their thymus removed if they’ve read that article.”

Then, in March, two consecutive studies collectively analyzed data from more than 31,000 people and expanded the findings to people with intact thymus that had naturally withered with age. A smaller thymus was found to correlate with a higher risk of death, as well as a higher risk of developing cancer and cardiovascular diseases such as heart failure.³. The second found that a smaller thymus is related to worse survival after receiving cancer immunotherapy⁴.

Some scientists point out that the studies only offer correlations. “When you look back, there are many pieces of the puzzle that simply cannot be known,” says Daniel Boffa, a thoracic surgeon at Yale University in New Haven, Connecticut. He and his colleagues conducted a study that found little difference in the risk of recurrent tumors between people who had all or part of their thymus removed to treat thymus cancer.⁸.

However, commercial and academic interest in the scam has increased. “Regrowth has become a big problem,” says Yousuke Takahama, a developmental biologist at the US National Cancer Institute in Bethesda, Maryland. “Just like restarting your computer, it’s like restarting our immune system.”