Castration, Longevity & Testosterone Therapy: What We Know

A new study published in Nature reported that sterilization and castration are associated with longer lifespans across a wide range of vertebrate species, including zoo animals, rodents, and some wild populations. The finding also appears to align with historical human data from Korean court eunuchs, who reportedly lived substantially longer than comparable men.

Most people are not going to interpret this as a practical intervention. The more relevant question is whether this kind of data should change how we think about testosterone, especially testosterone replacement therapy in aging men.

The answer is not simple. The animal findings are intriguing, but translating them into clinical decisions about testosterone requires caution.

What the study actually examined

The study combined two major data sources.

One source was zoo records. Many zoo animals are sterilized or placed on hormonal birth control, and zoos often keep detailed lifespan and cause-of-death records. The authors analyzed records from 117 types of zoo animals.

The second source was a broad review of the published literature on sterilization. The authors identified 71 studies covering 22 vertebrate species, ranging from fish to humans.

Across these datasets, male sterilization was associated with longer average lifespan. In male zoo animals, the lifespan increase was reported as roughly 10% on average, with similar patterns in the published studies.

What was most surprising in the results

If castration extended lifespan by reducing chronic disease, that would be a straightforward narrative. But the reported pattern was not centered on major chronic diseases such as cardiovascular disease and diabetes.

Instead, the largest apparent gains came from reduced mortality in a broad category of other causes, including rare or poorly categorized causes of death.

That creates a problem for interpretation. It suggests an effect, but it does not clearly identify the biological pathway driving the effect.

Why timing may matter

One of the more informative observations in the analysis was that the lifespan effect appeared strongest when castration occurred before puberty.

That detail matters because prepubertal castration changes developmental programming. It prevents the surge of testosterone that permanently shapes aspects of growth, body composition, and endocrine signaling.

The authors speculate that early removal of testosterone may alter growth hormone related pathways. These pathways are frequently linked to aging biology. The idea is that dampening growth signaling can sometimes shift physiology toward maintenance and repair.

This is conceptually consistent with other longevity research that focuses on growth and nutrient signaling pathways, including mTOR-related biology in animal models.

Two plausible mechanisms behind longer lifespan

The discussion points toward two broad categories of explanation.

Reduced risk behavior
Lower testosterone can reduce aggression, mating competition, territorial conflict, and injury risk in many species. In animal populations, fewer deaths from violence, accidents, or competition could move average lifespan.

Changes in longevity-related signaling
Separately, testosterone may influence growth signaling pathways that affect long-term aging trajectories, especially when altered before puberty. This could have downstream effects on metabolism, cellular stress responses, and repair systems.

These two mechanisms are not mutually exclusive. The challenge is that observational lifespan data cannot cleanly separate them.

What the human eunuch data suggests and what it cannot prove

The Korean eunuch data often comes up in discussions of testosterone and aging. Records from the Joseon Dynasty include enough birth and death dates to estimate lifespan in a subset of eunuchs, with reported averages that appear 14 to 19 years longer than non-castrated men of similar social class.

That is an attention-grabbing difference, but it is also an example of data that can be easy to overinterpret.

Key limitations include:

• Incomplete records, with birth and death dates available for only a subset of individuals

• Potential selection bias, where the individuals with complete records may not represent the full group

• Confounding factors related to living conditions, social protection, and occupational roles

Even if lifespan truly was longer, it does not automatically mean health outcomes were better in every domain. Other historical analyses of eunuch populations have found evidence consistent with osteoporosis, which fits expectations for very low lifelong testosterone.

Why low testosterone is not a free longevity strategy

The main problem with the simplistic takeaway, lower testosterone equals longer life, is that in modern clinical data low testosterone is often a marker of poor health.

Chronically low testosterone is associated with:

• Higher risk of type 2 diabetes

• Lower bone density and higher osteoporosis risk in older adults

• Higher all-cause mortality associations in multiple cohorts

These associations do not prove that low testosterone causes these outcomes. Low testosterone can be both a contributor and a signal of underlying metabolic dysfunction, chronic inflammation, or illness.

This is why the eunuch data and the cohort associations can appear to conflict. They are not measuring the same thing.

Prepubertal castration changes development. Late-life low testosterone in a modern adult is often intertwined with obesity, insulin resistance, sleep disruption, medication effects, and chronic disease.

What this means for testosterone replacement therapy

The practical question is not whether eliminating testosterone extends life. The practical question is when testosterone replacement therapy is appropriate, and what mistakes to avoid.

Two errors are common.

Treating a number instead of a person
A single testosterone value without symptoms and without context should not drive treatment.

Chasing supraphysiologic levels
Using testosterone to push levels above normal ranges can create risk without clear longevity benefit, and it can mask underlying lifestyle drivers that are often the true cause of suppression.

A more conservative clinical framing is:

• Address reversible causes first

• Confirm true hypogonadism with appropriate testing

• Treat when low testosterone is persistent and accompanied by symptoms

• Monitor carefully after therapy begins

Where lifestyle fits in

Lifestyle factors can strongly affect testosterone, and they are often the most productive starting point.

Weight and metabolic health
Obesity is one of the strongest predictors of low testosterone. Some studies report very large risk increases for low testosterone in men with BMI above 30.

Exercise
Resistance training and aerobic training both support healthier testosterone profiles and improve cardiometabolic risk, which matters regardless of testosterone.

Sleep
Insufficient sleep can lower testosterone, and short sleep has broader metabolic effects that worsen hormonal regulation.

In patients where weight loss is difficult, anti-obesity medications such as GLP-1 based therapies may be part of a plan, not because they directly replace testosterone, but because improving metabolic health can allow endogenous testosterone to recover.

Practical takeaways

• Sterilization is associated with longer lifespan across many vertebrate species, but mechanisms are not fully clear

• The strongest longevity effects appear when castration occurs before puberty, which limits direct relevance to adult clinical decisions

• Human eunuch data is intriguing but incomplete and potentially biased

• Low testosterone in modern adults often reflects underlying metabolic or health issues, not a longevity advantage

• Testosterone therapy may be appropriate for confirmed hypogonadism with symptoms, but it should not be used to chase high levels or ignore root causes

• Weight management, exercise, and sleep are first-line levers for supporting healthy testosterone and long-term health

Summary

The new Nature analysis adds weight to a long-observed pattern: reducing male sex hormone signaling can be associated with longer lifespan in many species. But the strongest effects occur when hormone exposure is altered early in life, and the human historical data is too limited to treat as definitive.

For modern health decisions, the more useful takeaway is not that lower testosterone is better. It is that testosterone is tightly linked to broader metabolic and lifestyle factors, and treatment should be individualized. Addressing obesity, inactivity, and poor sleep often does more for long-term health than trying to manipulate testosterone levels in isolation.

Research sources:
https://www.nature.com/articles/s41586-025-09836-9
https://www.cell.com/current-biology/fulltext/S0960-9822(12)00712-9
https://pmc.ncbi.nlm.nih.gov/articles/PMC9938530/
https://pmc.ncbi.nlm.nih.gov/articles/PMC5376477/
https://pmc.ncbi.nlm.nih.gov/articles/PMC3955324/
https://academic.oup.com/jcem/article-abstract/95/4/1810/2597149
https://academic.oup.com/jcem/article/103/5/1715/4939465