Oestrogen therapy produces measurable physiological changes in trans women, including reduced haemoglobin, lower muscle mass, and decreased strength. After two to three years of hormone therapy, endurance and aerobic performance measures fall within the female range. Skilled sports such as pickleball depend on practice and talent, not puberty history.
Why this question matters right now
Debates about trans women in sport have become loud, often ill-informed, and almost always more interested in scoring points than in following the physiology. I want to set out what the evidence actually shows, because trans women deserve to have this argued from the ground up, not from assumption, and because the people making policy decisions in sport need to know what they are actually deciding.
The conversation tends to collapse into a single claim: that male puberty confers a permanent, insurmountable advantage that no amount of hormone therapy can undo. That claim is not supported by the evidence. Some physiological differences persist into the early years of transition, and I will not pretend otherwise. But the picture by year two or three is substantially different from the picture people are imagining.
What oestrogen therapy actually does to the body
When trans women begin hormone therapy, oestrogen and anti-androgens work together to shift the body's physiology towards female norms. These are not cosmetic changes. They are systemic, measurable, and well-documented.
Haemoglobin, the protein in red blood cells that carries oxygen to working muscles during exercise, drops to female levels within roughly a year of starting therapy. This is significant because haemoglobin concentration is one of the primary determinants of aerobic capacity. Higher haemoglobin means more oxygen delivered per minute, which means more endurance. Once haemoglobin normalises, that particular advantage is gone.
Muscle mass decreases, and with it, absolute strength. This happens over one to three years. The rate varies between individuals, as it does with almost everything in medicine, but the direction is consistent. Trans women on oestrogen lose skeletal muscle, and their strength profiles shift accordingly.
Body fat redistribution also occurs, towards the pattern typical of women: more subcutaneous fat, less visceral fat, a shift in the ratio of fat to lean mass. This affects not just appearance but biomechanics, thermoregulation, and metabolic response to exercise.
What the evidence shows at two to three years
The finding that deserves to be heard clearly is this: after two years of hormone therapy, there is no meaningful difference between trans women and cis women in aerobic fitness, endurance, and running speed. By two to three years, endurance measures are in the female range.
That is not a campaigning claim. It is what the physiology shows when you follow it through long enough. Most of the studies that find residual advantage in trans women are measuring people early in their transition, sometimes within months of starting hormones, when the changes are still in progress. Using those numbers to justify lifetime exclusion is like measuring bone density at six months post-menopause and concluding nothing ever changes.
Strength is a more complex picture, and I want to be honest about that. Some studies suggest that certain absolute strength measures may take longer to fully equalise, particularly in activities that depend heavily on upper body power. This is a genuine area of ongoing research, and I would not wave it away. But "some strength measures may persist longer in some individuals in some sports" is a very different claim from "trans women always have an unfair advantage in all sport." The second claim, which is the one driving most policy, is not what the evidence says.
What surgery changes, and what it does not
Gender-affirming surgery, when it involves gonadectomy (removal of the testes), eliminates endogenous testosterone production. For trans women who have surgery, there is no longer any internal source of testosterone at all, and hormone therapy then maintains oestrogen levels without the need for anti-androgens.
Surgery does not, on its own, immediately change muscle mass, strength, or aerobic capacity. Those changes are driven by hormonal environment over time, not by the surgical event itself. What surgery does is make the hormonal environment more consistent and permanently removes the testosterone source, which can matter for long-term physiological change.
It is worth being clear that many trans women do not have surgery, and do not need to in order to meet the criteria of being a trans woman. Policy that requires surgery as a condition of sports participation is both medically illiterate and discriminatory. The relevant factor is hormonal environment, not surgical history.
Skills are not stored in testosterone
This brings me to something I find myself wanting to say whenever pickleball, or tennis, or badminton, or any skilled sport comes up in these conversations. People argue as though spending male puberty gives you a permanent, indelible advantage in hand-eye coordination, court positioning, tactical reading, shot selection. It does not.
You get better at those things through practice and talent. Full stop. A trans woman who is a brilliant pickleball player is brilliant because she has worked at it, because she reads the game well, because she has developed her shot under pressure, because she is good. Not because of what puberty she went through decades ago.
The sporting advantage that puberty-related hormonal exposure genuinely confers is physiological: oxygen-carrying capacity, muscle fibre recruitment, power output. Those are real, and those are what hormone therapy addresses over time. The skilled components of sport, the ones that make the difference between a competent player and an outstanding one in most recreational and many competitive contexts, are not on that list.
What this means for sports policy
I am not going to pretend this is simple at the elite level, where the margins are tiny and a small residual difference in power output could matter in certain events. That is a specific, narrow question, and it deserves a specific, evidence-based answer for each sport and each competitive level.
What it does not deserve is a blanket policy of exclusion applied to every trans woman in every sport at every level of competition, based on assumptions that the evidence does not support. Blanket exclusion of trans women from women's sport treats the possibility of advantage as a certainty, ignores the two-to-three-year hormonal timeline, and discards the dignity and wellbeing of trans women athletes for the sake of a hypothesis.
The sports organisations doing this right now are not doing it because the science compels them. They are doing it because it is politically expedient. That is worth naming plainly.
The evidence-based approach would set requirements around hormonal criteria, apply them proportionately to the level and nature of the competition, and revisit them as the science develops. That is not what is happening in most cases. What is happening is that trans women are being excluded, and the science is being invoked selectively to justify a conclusion that was reached before the evidence was consulted.
A note on reading the research
When you read studies on this topic, check a few things before accepting the headline. How long had participants been on hormone therapy? Studies at six months are not the same as studies at three years, and conflating them is common. What were they measuring? Absolute strength in a lab setting is not the same as performance in a real sport. What was the comparison group? Trans women compared to male athletes is a different question from trans women compared to female athletes who are their actual competitive peers.
The research in this area is growing, and it is not uniform. Some of it is well-designed and honest about its limitations. Some of it is driven by a prior conclusion. I do not think it serves anyone to pretend the science is fully settled, but I do think it serves trans women to insist that the science be read carefully and applied honestly, rather than weaponised selectively.
The honest summary of where the evidence stands: by two to three years of hormone therapy, the physiological markers most relevant to endurance sport are in the female range. Strength is a more complex picture, especially in power-based events, and research continues. Skill-based sports are a different question entirely, and the puberty-advantage argument does not hold there.
Trans women are women, and they deserve to play sport as women. The conversation about how to get the detail right in elite competition is worth having. The conversation about whether trans women belong in sport at all is not.
If there is a topic that you would like me to cover, just let Sammy know.
Written by Dr Helen Webberley, Gender Specialist and founder of GenderGP. Helen has spent decades working with trans people and advocating for evidence-based, compassionate gender-affirming care.
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