Let’s save you and me a lot of time. There is no scientific study on the hormonal effects of hormone replacement therapy on biological men athletes transition to transgender athletes.
“As trans women struggle to become involved in women’s sports, various governing bodies have introduced rules to mitigate any physical benefits they may have. But what those benefits are and how to counter them – and whether it’s necessary or even possible – has been hotly debated. The New Yorker
And to level the playing field of definitions, gender reflects a number of characteristics, defining and differentiating what we might also call feminine or masculine. These characteristics may be our social roles, perceptions, or the presence or absence of biological chromosomes, organs, or hormones. Our culture, written broadly, consistently redefines gender based on appearance (phenotype), chromosomes (genotype) and hormone levels, most recently in the case of runner Castor Seeds.
Our scientific thinking has tried to isolate on gender characteristics; In doing so, we ignore many other biological considerations, including the relationship of one hormone to another and how the effects of hormones can increase and decrease for longer periods of just a few weeks or months. A systematic review in the British Journal of Sports Medicine examines these equally important considerations.
In a meta-analysis, researchers reviewed 24 studies on ‘non-athletic transgender women’ undergoing sex-confirming hormone therapy (GAHT). This therapy usually includes an antitestosterone agent and estrogen to create a hormonal environment that reduces masculinization and promotes feminization. There is no evidence that it creates the hormonal environment and cyclicity of the woman.
The study examines all the measures considered for muscle strength and oxygen supply, probably necessary but not sufficient factors for athletic performance.  They focused on three measures for muscle mass, lean body mass, muscle strength and cross-sectional area and two endurance measurements, hemoglobin and hematocrit, indirect measures for the efficiency of transport and delivery of oxygenation . Testosterone increases all of these measures, so it is likely, and this is a crucial disclaimer, that antitestosterone therapy will reverse these effects.
As with all meta-analyzes, there was a mix of participants, GAHT therapies, study intervals, and species. More importantly,
- All studies involved a small number of participants, with the largest involving 277 participants.
- While most transgender women “achieved testosterone levels in the cisgender reference range for women,” several studies included had higher-than-usual levels, which may obscure the results.
Let us focus on qualitative findings instead of arguing about the importance of quantitative measures.
- All parameters of interest were reduced to varying degrees in women with GAHT therapy.
- At 12 months, both lean body mass, muscle mass, and muscle cross-sectional size were significantly reduced.
- But the loss of muscle strength varies from insignificant to 7%. “… after 12-36 months, the GAHT suggests that transgender women are likely to retain the advantage of power over cisgender women.”
- Hemoglobin and hematocrit, the supply of oxygen, which is essential for endurance, of transgenes are identical to those of ciswomen at four months.
What to conclude:
Again, it’s more complicated than we expected. Attempts to “mimic” a woman’s hormonal environment have failed to turn men into women in terms of muscle mass and strength. Transgenes may be more easily comparable to those in isolated aspects (hemoglobin and hematocrit) of the physiology of oxygen delivery. But the effect of extra muscle strength throws the key to the theory that hormone treatments can be “great equalizers.” This, as the author suggests, means that athletic activities that require endurance may have a different balance between transgender and gypsy than athletic activities that require strength.
“Although a level playing field in sport is illusory, it is important that there are opportunities for women to compete significantly in the women’s category. Whether transgender and cisgender women can engage in meaningful sports, even after GAHT, is a much debated issue.
 Additional factors include upper and lower body proportions, coordination, training, perception and experience abilities, among others.
 Hemoglobin is the molecule that carries oxygen to the body, and hematocrit refers to the ratio of red blood cells (which contain hemoglobin) to serum.
Source: How does the hormonal transition in transgender women change body composition, muscle strength and hemoglobin? Systematic review with emphasis on the consequences for sports participation. British Journal of Sports Medicine DOI: 10.1136 / bjsports-2020-103106