Study analysis · British Journal of Sports Medicine · 2021
Hormone therapy drops trans women’s blood oxygen markers to female levels in 4 months — but their strength may stay above cis women’s for at least 3 years.
A review of 24 studies found that gender-affirming hormone therapy in trans women lowers blood hemoglobin quickly and muscle strength/mass modestly, but a relative strength advantage over cis women may persist for up to 3 years; absolute changes were not reported.
Overview
What the study found
The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.
In simple terms
This study is a big summary of many smaller studies, not a single experiment. It shows that hormone therapy is linked to changes in body and blood, but because people weren't randomly assigned, we can't say hormone therapy definitely causes the changes. Also, the studies were mostly on non-athletes, so we can't be sure it applies to athletes.
What’s the bottom line?
Scientists looked at 24 studies to see how gender-affirming hormone therapy changes transgender women’s muscles and blood.
How strong is this study?
The researchers did a careful search and checked the quality of the studies they included. But the included studies were mostly small and not very strong, and they didn't randomly assign people to treatments. That means we should be cautious about trusting the results as proof of cause and effect.
40 / 100
- COI disclosure+40/40
- Data availabilitydata not shared
- Code availabilitycode not shared
0 / 100
- Randomizationnot randomized
- Blindingblinding unclear
- Control groupno control group
- Sample sizeno sample size reported
- Follow-upno follow-up reported
100 / 100
23 / 100
- P-values+15/15
- Effect sizeno effect size reported
- Confidence intervalsno confidence intervals
- Pre-registrationnot pre-registered
Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.
Where it sits
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 534 / 100
Probability of being correct
Systematic reviews and meta-analyses of cohort studies. They sit above a single cohort study but below a single randomized trial, because the underlying evidence is still observational.
This design cannot establish causation — the findings describe an association, not a cause. Observational systematic review of cohort and cross-sectional studies; no randomization, high risk of confounding, heterogeneous treatments and populations, and no control over variables. Cannot infer causal relationships.
COI Unknown
Could not determine conflict of interest status
No conflict of interest or funding statement was provided in the text; therefore, conflicts of interest cannot be assessed.
The provided text does not include a COI or funding section. The review notes risk of selective data reporting and publication bias due to data extraction from medical charts. No author affiliations or industry ties are disclosed.
Key takeaways
- 01
Hemoglobin/hematocrit dropped by 4.6%–14.0% relative to each person’s starting level and matched cisgender women after about 3–4 months.
- 02
After 12 months, some studies found relative drops in strength of 4.3%–7.1%, lean body mass of 3.0%–5.4%, and muscle area of 1.5%–9.7%.
- 03
Transwomen still had higher values than cisgender women even after 36 months.
- 04
Absolute changes were not reported.
- 05
These are relative percentage changes from each person’s own baseline, not absolute risk numbers.
- 06
The review does not report absolute changes or a baseline risk, so we cannot say how many extra or fewer cases this means.
- 07
For context, even after 12–36 months of hormone therapy, transwomen’s muscle mass and strength remained above cisgender women’s, which could matter for sport.
Surprising findings
- Hormone-naive trans women have lower lean body mass, muscle cross-sectional area, and handgrip strength than cisgender men despite similar testosterone levels.Testosterone is often assumed to be the main driver of male-typical strength advantages. This finding suggests other factors — possibly social, behavioral, or psychological — may contribute before any hormone therapy begins.
- After 36 months of hormone therapy, trans women’s strength, lean body mass, and muscle area values remain above cisgender women’s values.Many people assume hormone therapy fully equalizes muscle-related physiology within a year or two. This review suggests a relative strength advantage may persist for at least 3 years.
- Hemoglobin/hematocrit drops to cisgender female levels in about 3–4 months, while muscle changes take 12–36 months and may not fully equalize.It shows the body’s systems don’t transition on the same clock. Blood oxygen capacity changes quickly; muscle strength changes slowly.
- There are no studies in transgender athletes; the entire review is based on non-athletic trans women.This is one of the most heated debates in sport, yet the direct evidence from athletes is essentially absent.
Practical takeaways
For sports policymakers: consider sport-specific and time-course-specific eligibility rules, because blood oxygen markers change within months while muscle strength may take years to shift.
This review is based on non-athletic trans women, not athletes. It cannot predict individual performance or competitive outcomes.
medium confidenceFor trans women in sport: discuss individual monitoring with a doctor and sports body, since physiological changes vary widely and current evidence cannot guarantee a specific timeline for strength equalization.
No absolute changes were reported, and study quality was only moderate. Individual responses to hormone therapy differ.
low confidenceFor coaches and athletes: expect endurance-related markers like hemoglobin to drop relatively quickly, while strength and muscle mass may decline more slowly and may remain above cis female levels for at least 3 years.
These are relative changes from baseline, not absolute performance predictions. No direct athletic performance measures were included.
medium confidenceFor researchers: prioritize prospective studies in transgender athletes, with standardized hormone regimens, long follow-up, and sport-specific performance outcomes.
Such studies are ethically and logistically challenging, and current evidence is limited by small samples and overlapping cohorts.
high confidenceWhy this study matters
Blood changes fast, muscles change slow
Hemoglobin and hematocrit fell by 4.6%–14.0% relative to baseline and matched cisgender female levels after about 3–4 months of hormone therapy. In contrast, muscle strength, lean body mass, and muscle cross-sectional area showed relative decreases only after 12 months (strength 4.3%–7.1% in some studies, lean mass 3.0%–5.4%, muscle area 1.5%–9.7%). This means the body’s oxygen-carrying capacity and its muscle system follow very different timelines.
It challenges the idea that hormone therapy flips a single biological switch. Endurance-related blood markers change in months, while strength-related tissues may take years to shift — and may never fully match cis women.
Strength advantage may persist for 3 years
Even after 36 months of hormone therapy, trans women’s values for strength, lean body mass, and muscle area remained above those of cisgender women. The review suggests that strength may be well preserved in trans women during the first 3 years of hormone therapy. Effects beyond 36 months are unclear because of too little data.
This is the heart of the sports fairness debate: if a relative strength advantage remains for years, policies assuming rapid equalization may be incomplete.
Hormone-naive trans women already differ from cis men
Before starting hormone therapy, trans women had 6.4%–8.0% lower lean body mass, 6.0%–11.4% lower muscle cross-sectional area, and about 10%–14% lower handgrip strength than cisgender men — despite having similar testosterone levels. The study calls this disparity noteworthy and says explanations are unclear.
It busts the simple assumption that any male-typical strength advantage is purely testosterone-driven. Social, behavioral, and psychological factors may also shape athletic attributes.
No athlete data — the biggest limitation
The review is based on non-athletic trans women. The authors state: 'a major limitation in this area of research is the absence of studies in transgender athletes.' So we cannot directly say how these changes affect actual sport performance, training, or competition outcomes.
Many headlines treat this as definitive for sport, but the evidence comes from clinical populations, not athletes. That gap fuels both caution and controversy.
Endurance vs strength sports may need different rules
The review notes that hemoglobin/hematocrit changes follow a different time course than strength changes. It suggests sport-specific regulations for trans women in endurance versus strength sports may be needed. One small study of eight trans female distance runners found they were not more competitive in the female category after hormone therapy than they had been in the male category before.
It points toward a more nuanced policy approach: one-size-fits-all testosterone thresholds may not match the biology of different sports.
Body fat increases, muscle mass decreases
All studies reporting body composition found an increase in total body fat mass in trans women after hormone transition, alongside relative decreases in lean body mass. This shift in body composition matters for power-to-weight ratio and metabolic health.
It’s not just about losing muscle — gaining fat changes how the body moves and performs, especially in sports where weight matters.
Evidence quality is only moderate
All 24 studies were rated moderate quality. Small samples, short follow-up, heterogeneous hormone regimens, overlapping ENIGI cohorts, and no meta-analysis limit confidence. The paper also has published corrections/errata, so readers should check updated notices.
It’s a reminder that even a widely cited review can be less definitive than it sounds — and that science communication should reflect uncertainty.
Want the whole report?
Detailed mode opens the full scientific breakdown — every score component, the methodology, conflicts of interest, the evidence analysis behind each claim, and the raw study data.
Overview
What the study found
The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Scientists looked at 24 studies to see how gender-affirming hormone therapy changes transgender women’s muscles and blood.
Research results
Hemoglobin/hematocrit dropped by 4.6%–14.0% relative to each person’s starting level and matched cisgender women after about 3–4 months. After 12 months, some studies found relative drops in strength of 4.3%–7.1%, lean body mass of 3.0%–5.4%, and muscle area of 1.5%–9.7%. Transwomen still had higher values than cisgender women even after 36 months. Absolute changes were not reported.
What this means - more context
These are relative percentage changes from each person’s own baseline, not absolute risk numbers. The review does not report absolute changes or a baseline risk, so we cannot say how many extra or fewer cases this means. For context, even after 12–36 months of hormone therapy, transwomen’s muscle mass and strength remained above cisgender women’s, which could matter for sport.
Systematic review of how testosterone-suppressing gender-affirming hormone therapy (GAHT) changes lean body mass, muscle cross-sectional area, muscular strength, and hemoglobin/hematocrit in transgender women, with implications for sport participation.
Twenty-four studies (1999–2020) were reviewed. GAHT was associated with relative decreases in all measured parameters, with different time courses. Hemoglobin/hematocrit fell 4.6%–14.0% relative to baseline and reached cisgender female levels after about 3–4 months. After 12 months, relative decreases were seen in strength (4.3%–7.1% in some studies), lean body mass (3.0%–5.4%), and muscle cross-sectional area (1.5%–9.7%). Despite these relative decreases, transwomen’s values remained above cisgender women’s even after 36 months, suggesting a preserved strength advantage during the first 3 years; effects beyond 36 months were unclear. Note: this study has published corrections/errata; readers should check the correction notices for updated information. Absolute risk or absolute changes were not reported in this review.
Methods Used
Systematic review following PRISMA guidelines. Searched BioMed Central, PubMed, Scopus, and Web of Science in April 2020 for English papers from 1999 to 2020. Included transwomen studies measuring at least one of: lean body mass, muscle cross-sectional area, muscular strength, or hemoglobin/hematocrit. Quality assessed with the Effective Public Health Practice Project tool; 24 studies included. No meta-analysis was possible due to heterogeneity and overlapping ENIGI cohorts.
Main Finding
In transgender women, GAHT rapidly lowers hemoglobin/hematocrit by 4.6%–14.0% relative to baseline, reaching cisgender female levels by about 3–4 months. By 12 months, relative decreases occur in strength (4.3%–7.1% in some studies), lean body mass (3.0%–5.4%), and muscle cross-sectional area (1.5%–9.7%). However, values remain above cisgender women’s even after 36 months, suggesting a preserved relative strength advantage during the first 3 years; longer-term effects are unclear. Absolute risk/change was not reported.
Confidence Level
Moderate: all 24 studies were rated moderate quality by EPHPP, and results were largely consistent across different designs. However, limitations include small samples, short follow-up, heterogeneous hormone regimens, overlapping cohorts, no athlete-specific data, and no meta-analysis. Corrections/errata exist, so readers should check the notices.
Study Flags
Red Flags
- •Corrections/errata exist; check correction notices for updated information.
- •No studies in transgender athletes; evidence comes from non-athletic transwomen, limiting sport-performance conclusions.
- •Small samples, short follow-up, heterogeneous hormone regimens, and overlapping ENIGI cohorts precluded meta-analysis; all studies rated only moderate quality.
Surprising Findings
Hormone-naive trans women have lower lean body mass, muscle cross-sectional area, and handgrip strength than cisgender men despite similar testosterone levels.
Testosterone is often assumed to be the main driver of male-typical strength advantages. This finding suggests other factors — possibly social, behavioral, or psychological — may contribute before any hormone therapy begins.
Practical Takeaways
For sports policymakers: consider sport-specific and time-course-specific eligibility rules, because blood oxygen markers change within months while muscle strength may take years to shift.
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 534 / 100
Probability of being correct
Systematic reviews and meta-analyses of cohort studies. They sit above a single cohort study but below a single randomized trial, because the underlying evidence is still observational.
Systematic Review
Subject
Lower probability
on the GRADE evidence scale
This study is a big summary of many smaller studies, not a single experiment. It shows that hormone therapy is linked to changes in body and blood, but because people weren't randomly assigned, we can't say hormone therapy definitely causes the changes. Also, the studies were mostly on non-athletes, so we can't be sure it applies to athletes.
Strengths
- Systematic review following PRISMA guidelines
- Comprehensive search of four databases
- Quality assessment using EPHPP tool
Weaknesses
- No randomized controlled trials included
- All included studies rated moderate quality
- High heterogeneity in hormone regimens, populations, and outcome measures
Methodology
Evidence Keywords
Statistical Reporting
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Scientists looked at 24 studies to see how gender-affirming hormone therapy changes transgender women’s muscles and blood.
Research results
Hemoglobin/hematocrit dropped by 4.6%–14.0% relative to each person’s starting level and matched cisgender women after about 3–4 months. After 12 months, some studies found relative drops in strength of 4.3%–7.1%, lean body mass of 3.0%–5.4%, and muscle area of 1.5%–9.7%. Transwomen still had higher values than cisgender women even after 36 months. Absolute changes were not reported.
What this means - more context
These are relative percentage changes from each person’s own baseline, not absolute risk numbers. The review does not report absolute changes or a baseline risk, so we cannot say how many extra or fewer cases this means. For context, even after 12–36 months of hormone therapy, transwomen’s muscle mass and strength remained above cisgender women’s, which could matter for sport.
Systematic review of how testosterone-suppressing gender-affirming hormone therapy (GAHT) changes lean body mass, muscle cross-sectional area, muscular strength, and hemoglobin/hematocrit in transgender women, with implications for sport participation.
Twenty-four studies (1999–2020) were reviewed. GAHT was associated with relative decreases in all measured parameters, with different time courses. Hemoglobin/hematocrit fell 4.6%–14.0% relative to baseline and reached cisgender female levels after about 3–4 months. After 12 months, relative decreases were seen in strength (4.3%–7.1% in some studies), lean body mass (3.0%–5.4%), and muscle cross-sectional area (1.5%–9.7%). Despite these relative decreases, transwomen’s values remained above cisgender women’s even after 36 months, suggesting a preserved strength advantage during the first 3 years; effects beyond 36 months were unclear. Note: this study has published corrections/errata; readers should check the correction notices for updated information. Absolute risk or absolute changes were not reported in this review.
Methods Used
Systematic review following PRISMA guidelines. Searched BioMed Central, PubMed, Scopus, and Web of Science in April 2020 for English papers from 1999 to 2020. Included transwomen studies measuring at least one of: lean body mass, muscle cross-sectional area, muscular strength, or hemoglobin/hematocrit. Quality assessed with the Effective Public Health Practice Project tool; 24 studies included. No meta-analysis was possible due to heterogeneity and overlapping ENIGI cohorts.
Main Finding
In transgender women, GAHT rapidly lowers hemoglobin/hematocrit by 4.6%–14.0% relative to baseline, reaching cisgender female levels by about 3–4 months. By 12 months, relative decreases occur in strength (4.3%–7.1% in some studies), lean body mass (3.0%–5.4%), and muscle cross-sectional area (1.5%–9.7%). However, values remain above cisgender women’s even after 36 months, suggesting a preserved relative strength advantage during the first 3 years; longer-term effects are unclear. Absolute risk/change was not reported.
Confidence Level
Moderate: all 24 studies were rated moderate quality by EPHPP, and results were largely consistent across different designs. However, limitations include small samples, short follow-up, heterogeneous hormone regimens, overlapping cohorts, no athlete-specific data, and no meta-analysis. Corrections/errata exist, so readers should check the notices.
Study Flags
Red Flags
- •Corrections/errata exist; check correction notices for updated information.
- •No studies in transgender athletes; evidence comes from non-athletic transwomen, limiting sport-performance conclusions.
- •Small samples, short follow-up, heterogeneous hormone regimens, and overlapping ENIGI cohorts precluded meta-analysis; all studies rated only moderate quality.
Surprising Findings
Hormone-naive trans women have lower lean body mass, muscle cross-sectional area, and handgrip strength than cisgender men despite similar testosterone levels.
Testosterone is often assumed to be the main driver of male-typical strength advantages. This finding suggests other factors — possibly social, behavioral, or psychological — may contribute before any hormone therapy begins.
Practical Takeaways
For sports policymakers: consider sport-specific and time-course-specific eligibility rules, because blood oxygen markers change within months while muscle strength may take years to shift.
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 534 / 100
Probability of being correct
Systematic reviews and meta-analyses of cohort studies. They sit above a single cohort study but below a single randomized trial, because the underlying evidence is still observational.
Systematic Review
Subject
Lower probability
on the GRADE evidence scale
This study is a big summary of many smaller studies, not a single experiment. It shows that hormone therapy is linked to changes in body and blood, but because people weren't randomly assigned, we can't say hormone therapy definitely causes the changes. Also, the studies were mostly on non-athletes, so we can't be sure it applies to athletes.
Strengths
- Systematic review following PRISMA guidelines
- Comprehensive search of four databases
- Quality assessment using EPHPP tool
Weaknesses
- No randomized controlled trials included
- All included studies rated moderate quality
- High heterogeneity in hormone regimens, populations, and outcome measures
Methodology
Evidence Keywords
Statistical Reporting
Scoring
How strong is this study?
The researchers did a careful search and checked the quality of the studies they included. But the included studies were mostly small and not very strong, and they didn't randomly assign people to treatments. That means we should be cautious about trusting the results as proof of cause and effect.
40 / 100
- COI disclosure+40/40
- Data availabilitydata not shared
- Code availabilitycode not shared
0 / 100
- Randomizationnot randomized
- Blindingblinding unclear
- Control groupno control group
- Sample sizeno sample size reported
- Follow-upno follow-up reported
100 / 100
23 / 100
- P-values+15/15
- Effect sizeno effect size reported
- Confidence intervalsno confidence intervals
- Pre-registrationnot pre-registered
Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.
Where it sits
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 534 / 100
Probability of being correct
Systematic reviews and meta-analyses of cohort studies. They sit above a single cohort study but below a single randomized trial, because the underlying evidence is still observational.
This design cannot establish causation — the findings describe an association, not a cause. Observational systematic review of cohort and cross-sectional studies; no randomization, high risk of confounding, heterogeneous treatments and populations, and no control over variables. Cannot infer causal relationships.
COI Unknown
Could not determine conflict of interest status
No conflict of interest or funding statement was provided in the text; therefore, conflicts of interest cannot be assessed.
The provided text does not include a COI or funding section. The review notes risk of selective data reporting and publication bias due to data extraction from medical charts. No author affiliations or industry ties are disclosed.
Standing
The people behind it
The researchers who wrote the study this analysis is built on.
Authored by
5 researchersIf this is your work, this is how we attribute it on Fit Body Science. Joanna Harper is listed as the lead author.