Sports Medicine (2021) 51:199–214
Transgender Women intheFemale Category ofSport: Perspectives
onTestosterone Suppression andPerformance Advantage
Published online: 8 December 2020
© The Author(s) 2020
Males enjoy physical performance advantages over females within competitive sport. The sex-based segregation into male
and female sporting categories does not account for transgender persons who experience incongruence between their bio-
logical sex and their experienced gender identity. Accordingly, the International Olympic Committee (IOC) determined
criteria by which a transgender woman may be eligible to compete in the female category, requiring total serum testosterone
levels to be suppressed below 10nmol/L for at least 12months prior to and during competition. Whether this regulation
removes the male performance advantage has not been scrutinized. Here, we review how diﬀerences in biological charac-
teristics between biological males and females aﬀect sporting performance and assess whether evidence exists to support
the assumption that testosterone suppression in transgender women removes the male performance advantage and thus
delivers fair and safe competition. We report that the performance gap between males and females becomes signiﬁcant at
puberty and often amounts to 10–50% depending on sport. The performance gap is more pronounced in sporting activities
relying on muscle mass and explosive strength, particularly in the upper body. Longitudinal studies examining the eﬀects of
testosterone suppression on muscle mass and strength in transgender women consistently show very modest changes, where
the loss of lean body mass, muscle area and strength typically amounts to approximately 5% after 12months of treatment.
Thus, the muscular advantage enjoyed by transgender women is only minimally reduced when testosterone is suppressed.
Sports organizations should consider this evidence when reassessing current policies regarding participation of transgender
women in the female category of sport.
Given that biological males experience a substantial per-
formance advantage over females in most sports, there
is currently a debate whether inclusion of transgender
women in the female category of sports would compro-
mise the objective of fair and safe competition.
Here, we report that current evidence shows the biologi-
cal advantage, most notably in terms of muscle mass and
strength, conferred by male puberty and thus enjoyed
by most transgender women is only minimally reduced
when testosterone is suppressed as per current sporting
guidelines for transgender athletes.
This evidence is relevant for policies regarding partici-
pation of transgender women in the female category of
Supplementary Information The online version contains
supplementary material available at https ://doi.org/10.1007/s4027
* Tommy R. Lundberg
1 Faculty ofBiology, Medicine andHealth, University
ofManchester, Manchester, UK
2 Department ofLaboratory Medicine/ANA Futura, Division
ofClinical Physiology, Karolinska Institutet, Alfred Nobles
Allé 8B, Huddinge, 14152Stockholm, Sweden
3 Unit ofClinical Physiology, Karolinska University Hospital,
200 E.N.Hilton, T.R.Lundberg
Sporting performance is strongly inﬂuenced by a range of
physiological factors, including muscle force and power-
producing capacity, anthropometric characteristics, cardi-
orespiratory capacity and metabolic factors [1, 2]. Many
of these physiological factors diﬀer signiﬁcantly between
biological males and females as a result of genetic diﬀer-
ences and androgen-directed development of secondary
sex characteristics [3, 4]. This confers large sporting per-
formance advantages on biological males over females .
When comparing athletes who compete directly against
one another, such as elite or comparable levels of school-
aged athletes, the physiological advantages conferred by
biological sex appear, on assessment of performance data,
insurmountable. Further, in sports where contact, collision
or combat are important for gameplay, widely diﬀerent
physiological attributes may create safety and athlete wel-
fare concerns, necessitating not only segregation of sport
into male and female categories, but also, for example,
into weight and age classes. Thus, to ensure that both men
and women can enjoy sport in terms of fairness, safety and
inclusivity, most sports are divided, in the ﬁrst instance,
into male and female categories.
Segregating sports by biological sex does not account
for transgender persons who experience incongruence
betweentheir biological sex and their experienced gen-
der identity, and whose legal sex may be diﬀerent to that
recorded at birth [6, 7]. More speciﬁcally, transgender
women (observed at birth as biologically male but identi-
fying as women) may, before or after cross-hormone treat-
ment, wish to compete in the female category. This has
raised concerns about fairness and safety within female
competition, and the issue of how to fairly and safely
accommodate transgender persons in sport has been sub-
ject to much discussion [6–13].
The current International Olympic Committee (IOC)
policy  on transgender athletes states that “it is neces-
sary to ensure insofar as possible that trans athletes are not
excluded from the opportunity to participate in sporting
competition”. Yet the policy also states that “the overrid-
ing sporting objective is and remains the guarantee of fair
competition”. As these goals may be seen as conﬂicting if
male performance advantages are carried through to com-
petition in the female category, the IOC concludes that
“restrictions on participation are appropriate to the extent
that they are necessary and proportionate to the achieve-
ment of that objective”.
Accordingly, the IOC determined criteria by which
transgender women may be eligible to compete in the
female category. These include a solemn declaration that
her gender identity is female and the maintenance of total
serum testosterone levels below 10nmol/L for at least
12months prior to competing and during competition .
Whilst the scientiﬁc basis for this testosterone threshold
was not openly communicated by the IOC, it is surmised
that the IOC believed this testosterone criterion suﬃcient
to reduce the sporting advantages of biological males over
females and deliver fair and safe competition within the
Several studies have examined the eﬀects of testosterone
suppression on the changing biology, physiology and perfor-
mance markers of transgender women. In this review, we aim
to assess whether evidenceexists to support the assumption
that testosterone suppression in transgender women removes
these advantages. To achieve this aim, we ﬁrst review the
diﬀerences in biologicalcharacteristics between biological
males and females, and examine how those diﬀerences aﬀect
sporting performance. We then evaluate the studies that have
measured elements of performance and physical capacity
following testosterone suppression in untrained transgender
women, and discuss the relevance of these ﬁndings to the
supposition of fairness and safety (i.e. removal of the male
performance advantage) as per current sporting guidelines.
2 The Biological Basis forSporting
Performance Advantages inMales
The physical divergence between males and females begins
during early embryogenesis, when bipotential gonads are
triggered to differentiate into testes or ovaries, the tis-
sues that will produce sperm in males and ova in females,
respectively . Gonad diﬀerentiation into testes or ovaries
determines, via the speciﬁc hormone milieu each generates,
downstream in utero reproductive anatomy development
, producing male or female body plans. We note that in
rare instances, diﬀerences in sex development (DSDs) occur
and the typical progression of male or female development is
disrupted . The categorisation of such athletes is beyond
the scope of this review, and the impact of individual DSDs
on sporting performance must be considered on their own
In early childhood, prior to puberty, sporting participation
prioritises team play and the development of fundamental
motor and social skills, and is sometimes mixed sex. Athletic
performance diﬀerences between males and females prior to
puberty are often considered inconsequential or relatively
small . Nonetheless, pre-puberty performance diﬀer-
ences are not unequivocally negligible, and could be medi-
ated, to some extent, by genetic factors and/or activation of
the hypothalamic–pituitary–gonadal axis during the neonatal
period, sometimes referred to as “minipuberty”. For exam-
ple, some 6500 genes are diﬀerentially expressed between
males and females  with an estimated 3000 sex-speciﬁc
Eﬀects of Testosterone Suppression in Transgender Women
diﬀerences in skeletal muscle likely to inﬂuence composition
and function beyond the eﬀects of androgenisation , while
increased testosterone during minipuberty in males aged
1–6months may be correlated with higher growth velocity
and an “imprinting eﬀect” on BMI and bodyweight [20, 21].
An extensive review of ﬁtness data from over 85,000 Aus-
tralian children aged 9–17years old showed that, compared
with 9-year-old females, 9-year-old males were faster over
short sprints (9.8%) and 1 mile (16.6%), could jump 9.5%
further from a standing start (a test of explosive power),
could complete 33% more push-ups in 30s and had 13.8%
stronger grip . Male advantage of a similar magnitude
was detected in a study of Greek children, where, compared
with 6-year-old females, 6-year-old males completed 16.6%
more shuttle runs in a given time and could jump 9.7%
further from a standing position . In terms of aerobic
capacity, 6- to 7-year-old males have been shown to have a
higher absolute and relative (to body mass) VO2max than 6- to
7-year-old females .Nonetheless, while some biological
sex diﬀerences, probably genetic in origin, are measurable
and aﬀect performance pre-puberty, we consider the eﬀect of
androgenizing puberty more inﬂuential on performance, and
have focused our analysis on musculoskeletal diﬀerences
Secondary sex characteristics that develop during puberty
have evolved under sexual selection pressures to improve
reproductive ﬁtness and thus generate anatomical divergence
beyond the reproductive system, leading to adult body types
that are measurably diﬀerent between sexes. This phenom-
enon is known as sex dimorphism. During puberty, testes-
derived testosterone levels increase 20-fold in males, but
remain low in females, resulting in circulating testosterone
concentrations at least 15 times higher in males than in
females of any age [4, 25]. Testosterone in males induces
changes in muscle mass, strength, anthropometric variables
and hemoglobin levels , as part of the range of sexually
dimorphic characteristics observed in humans.
Broadly, males are bigger and stronger than females. It
follows that, within competitive sport, males enjoy signiﬁ-
cant performance advantages over females, predicated on
the superior physical capacity developed during puberty
in response to testosterone. Thus, the biological eﬀects of
elevated pubertal testosterone are primarily responsible for
driving the divergence of athletic performances between
males and females . It is acknowledged that this diver-
gence has been compounded historically by a lag in the cul-
tural acceptance of, and ﬁnancial provision for, females in
sport that may have had implications for the rate of improve-
ment in athletic performance in females. Yet, since the
1990s, the diﬀerence in performance records between males
and females has been relatively stable, suggesting that bio-
logical diﬀerences created by androgenization explain most
of the male advantage, and are insurmountable [5, 26, 27].
Table1 outlines physical attributes that are major parame-
ters underpinning the male performance advantage [28–38].
Males have: larger and denser muscle mass, and stiﬀer con-
nective tissue, with associated capacity to exert greater mus-
cular force more rapidly and eﬃciently; reduced fat mass,
and diﬀerent distribution of body fat and lean muscle mass,
which increases power to weight ratios and upper to lower
limb strength in sports where this may be a crucial determi-
nant of success; longer and larger skeletal structure, which
creates advantages in sports where levers inﬂuence force
application, where longer limb/digit length is favorable, and
where height, mass and proportions are directly responsi-
ble for performance capacity; superior cardiovascular and
respiratory function, with larger blood and heart volumes,
higher hemoglobin concentration, greater cross-sectional
area of the trachea and lower oxygen cost of respiration [3, 4,
39, 40]. Of course, diﬀerent sports select for diﬀerent physi-
ological characteristics—an advantage in one discipline may
be neutral or even a disadvantage in another—but examina-
tion of a variety of record and performance metrics in any
discipline reveals there are few sporting disciplines where
males do not possess performance advantage over females
as a result of the physiological characteristics aﬀected by
3 Sports Performance Dierences Between
3.1 An Overview ofElite Adult Athletes
A comparison of adult elite male and female achievements
in sporting activities can quantify the extent of the male per-
formance advantage. We searched publicly available sports
federation databases and/or tournament/competition records
to identify sporting metrics in various events and disciplines,
and calculated the performance of males relative to females.
Although not an exhaustive list, examples of performance
gaps in a range of sports with various durations, physiologi-
cal performance determinants, skill components and force
requirements are shown in Fig.1.
The smallest performance gaps were seen in rowing,
swimming and running (11–13%), with low variation across
individual events within each of those categories. The
performance gap increases to an average of 16% in track
cycling, with higher variation across events (from 9% in the
4000m team pursuit to 24% in the ﬂying 500m time trial).
The average performance gap is 18% in jumping events
(long jump, high jump and triple jump). Performance dif-
ferences larger than 20% are generally present when consid-
ering sports and activities that involve extensive upper body
contributions. The gap between fastest recorded tennis serve
202 E.N.Hilton, T.R.Lundberg
Table 1 Selected physical
diﬀerence between untrained/
moderately trained males and
females. Female levels are set as
the reference value
Variable Magnitude of sex diﬀerence
Lean body mass 45 Lee etal. 
Fat% − 30
Lower body 33 Janssen etal. 
Upper body 40
Grip strength 57 Bohannon etal. 
Knee extension peak torque 54 Neder etal. 
Anthropometry and bone geometry
Femur length 9.4 Jantz etal. 
Humerus length 12.0 Brinckmann etal. 
Radius length 14.6
Pelvic width relative to pelvis height − 6.1
Force 83 Lepley etal. 
Absolute values 50 Pate etal. 
Relative values 25
Pulmonary ventilation (maximal) 48 Åstrand etal. 
Left ventricular mass 31 Åstrand etal. 
Cardiac output (rest) 22 Best etal. 
Cardiac output (maximal) 30 Tong etal. 
Stroke volume (rest) 43
Stroke volume (maximal) 34
Hemoglobin concentration 11
Fig. 1 The male performance
advantage over females across
various selected sporting
disciplines. The female level
is set to 100%. In sport events
with multiple disciplines, the
male value has been averaged
across disciplines, and the error
bars represent the range of the
advantage. The metrics were
compiled from publicly avail-
able sports federation databases
records. MTB mountain bike
Eﬀects of Testosterone Suppression in Transgender Women
is 20%, while the gaps between fastest recorded baseball
pitches and ﬁeld hockey drag ﬂicks exceed 50%.
Sports performance relies to some degree on the magni-
tude, speed and repeatability of force application, and, with
respect to the speed of force production (power), vertical
jump performance is on average 33% greater in elite men
than women, with diﬀerences ranging from 27.8% for endur-
ance sports to in excess of 40% for precision and combat
sports . Because implement mass diﬀers, direct com-
parisons are not possible in throwing events in track and
ﬁeld athletics. However, the performance gap is known to
be substantial, and throwing represents the widest sex dif-
ference in motor performance from an early age . In
Olympic javelin throwers, this is manifested in diﬀerences
in the peak linear velocities of the shoulder, wrist, elbow
and hand, all of which are 13–21% higher for male athletes
compared with females .
The increasing performance gap between males and
females as upper body strength becomes more critical for
performance is likely explained to a large extent by the
observation that males have disproportionately greater
strength in their upper compared to lower body, while
females show the inverse [44, 45]. This diﬀerent distribution
of strength compounds the general advantage of increased
muscle mass in upper body dominant disciplines. Males also
have longer arms than females, which allows greater torque
production from the arm lever when, for example, throwing
a ball, punching or pushing.
3.2 Olympic Weightlifting
In Olympic weightlifting, where weight categories dif-
fer between males and females, the performance gap is
between 31 and 37% across the range of competitive body
weights between 1998 and 2020 (Fig.1). It is important to
note that at all weight categories below the top/open cate-
gory, performances are produced within weight categories
with an upper limit, where strength can be correlated with
“ﬁghting weight”, and we focused our analysis of perfor-
mance gaps in these categories.
To explore strength–mass relationships further, we
compared Olympic weightlifting data between equiva-
lent weight categories which, to some extent, limit athlete
height, to examine the hypothesis that male performance
advantage may be largely (or even wholly) mediated by
increased height and lever-derived advantages (Table2).
Between 1998 and 2018, a 69kg category was common
to both males and females, with the male record holder
(69kg, 1.68m) lifting a combined weight 30.1% heavier
than the female record holder (69kg, 1.64m). Weight cate-
gory changes in 2019 removed the common 69kg category
and created a common 55kg category. The current male
record holder (55kg, 1.52m) lifts 29.5% heavier than the
female record holder (55kg, 1.52m). These comparisons
demonstrate that males are approximately 30% stronger
than females of equivalent stature and mass. However,
importantly, male vs. female weightlifting performance
gaps increase with increasing bodyweight. For example, in
the top/open weight category of Olympic weightlifting, in
the absence of weight (and associated height) limits, maxi-
mum male lifting strength exceeds female lifting strength
by nearly 40%. This is further manifested in powerlift-
ing, where the male record (total of squat, bench press
and deadlift) is 65% higher than the female record in the
open weight category of the World Open Classic Records.
Further analysis of Olympic weightlifting data shows that
the 55-kg male record holder is 6.5% stronger than the
69-kg female record holder (294kg vs 276kg), and that
the 69-kg male record is 3.2% higher than the record held
in the female open category by a 108-kg female (359kg vs
348kg). This Olympic weightlifting analysis reveals key
diﬀerences between male and female strength capacity.
It shows that, even after adjustment for mass, biological
males are signiﬁcantly stronger (30%) than females, and
Table 2 Olympic weightlifting
data between equivalent male–
female and top/open weight
F female, M male
Sex Weight (kg) Height (m) Combined
record (kg) Strength to
weight ratio Relative
2019 record in the 55kg weight-limited category
Liao Qiuyun F 55 1.52 227 4.13
Om Yun-chol M 55 1.52 294 5.35 29.5
1998–2018 record in the 69-kg weight-limited category
Oxsana Slivenko F 69 1.64 276 4.00
Liao Hui M 69 1.68 359 5.20 30.1
Comparative performances for top/open categories (all time heaviest combined lifts)
Tatiana Kashirina F 108 1.77 348 3.22
Lasha Talakhadze M 168 1.97 484 2.88 39.1
204 E.N.Hilton, T.R.Lundberg
that females who are 60% heavier than males do not over-
come these strength deﬁcits.
3.3 Perspectives onElite Athlete Performance
Figure1 illustrates the performance gap between adult
elite males and adult elite females across various sporting
disciplines and activities. The translation of these advan-
tages, assessed as the performance diﬀerence between
the very best males and very best females, are signiﬁcant
when extended and applied to larger populations. In run-
ning events, for example, where the male–female gap is
approximately 11%, it follows that many thousands of
males are faster than the very best females. For example,
approximately 10,000 males have personal best times that
are faster than the current Olympic 100m female cham-
pion (World Athletics, personal communication, July
2019). This has also been described elsewhere [46, 47],
and illustrates the true eﬀect of an 11% typical diﬀerence
on population comparisons between males and females.
This is further apparent upon examination of selected jun-
ior male records, which surpass adult elite female perfor-
mances by the age of 14–15years (Table3), demonstrat-
ing superior male athletic performance over elite females
within a few years of the onset of puberty.
These data overwhelmingly conﬁrm that testosterone-
driven puberty, as the driving force of development of
male secondary sex characteristics, underpins sporting
advantages that are so large no female could reasonably
hope to succeed without sex segregation in most sporting
competitions. To ensure, in light of these analyses, that
female athletes can be included in sporting competitions in
a fair and safe manner, most sports have a female category
the purpose of which is the protection of both fairness
and, in some sports, safety/welfare of athletes who do not
beneﬁt from the physiological changes induced by male
levels of testosterone from puberty onwards.
3.4 Performance Diﬀerences inNon‑elite
The male performance advantages described above in ath-
letic cohorts are similar in magnitude in untrained people.
Even when expressed relative to fat-free weight, VO2max is
12–15% higher in males than in females . Records of
lower-limb muscle strength reveal a consistent 50% diﬀer-
ence in peak torque between males and females across the
lifespan . Hubal etal.  tested 342 women and 243
men for isometric (maximal voluntary contraction) and
dynamic strength (one-repetition maximum; 1RM) of the
elbow ﬂexor muscles and performed magnetic resonance
imaging (MRI) of the biceps brachii to determine cross-
sectional area. The males had 57% greater muscle size,
109% greater isometric strength, and 89% greater 1RM
strength than age-matched females. This reinforces the
ﬁnding in athletic cohorts that sex diﬀerences in muscle
size and strength are more pronounced in the upper body.
Recently, sexual dimorphism in arm force and power
was investigated in a punch motion in moderately-trained
individuals . The power produced during a punch was
162% greater in males than in females, and the least pow-
erful man produced more power than the most powerful
woman. This highlights that sex diﬀerences in parameters
such as mass, strength and speed may combine to pro-
duce even larger sex diﬀerences in sport-speciﬁc actions,
which often are a product of how various physical capaci-
ties combine. For example, power production is the prod-
uct of force and velocity, and momentum is deﬁned as
mass multiplied by velocity. The momentum and kinetic
energy that can be transferred to another object, such as
during a tackle or punch in collision and combat sports
are, therefore, dictated by: the mass; force to accelerate
that mass, and; resultant velocity attained by that mass.
As there is a male advantage for each of these factors, the
net result is likely synergistic in a sport-speciﬁc action,
such as a tackle or a throw, that widely surpasses the sum
of individual magnitudes of advantage in isolated ﬁtness
variables. Indeed, already at 17years of age, the average
male throws a ball further than 99% of 17-year-old females
, despite no single variable (arm length, muscle mass
etc.) reaching this numerical advantage. Similarly, punch
power is 162% greater in men than women even though no
single parameter that produces punching actions achieves
this magnitude of diﬀerence .
Table 3 Selected junior male records in comparison with adult elite
Time format: minutes:seconds.hundredths of a second
Event Schoolboy male record Elite female
100m 10.20 (age 15) 10.49
800m 1:51.23 (age 14) 1:53.28
1500m 3:48.37 (age 14) 3:50.07
Long jump 7.85m (age 15) 7.52m
Discus throw 77.68m (age 15) 76.80m
Eﬀects of Testosterone Suppression in Transgender Women
4 Is theMale Performance Advantage
Lost whenTestosterone isSuppressed
The current IOC criteria for inclusion of transgender
women in female sports categories require testosterone
suppression below 10nmol/L for 12months prior to and
during competition. Given the IOC’s stated position that
the “overriding sporting objective is and remains the guar-
antee of fair competition” , it is reasonable to assume
that the rationale for this requirement is that it reduces
the male performance advantages described previously to
an acceptable degree, thus permitting fair and safe com-
petition. To determine whether this medical intervention
is suﬃcient to remove (or reduce) the male performance
advantage, which we described above, we performed a
systematic search of the scientiﬁc literature addressing
anthropometric and muscle characteristics of transgender
women. Search terms and ﬁltering of peer-reviewed data
are given in Supplementary TableS1.
Given its importance for the general health of the transgen-
der population, there are multiple studies of bone health,
and reviews of these data. To summarise, transgender
women often have low baseline (pre-intervention) bone
mineral density (BMD), attributed to low levels of physi-
cal activity, especially weight-bearing exercise, and low
vitamin D levels [52, 53]. However, transgender women
generally maintain bone mass over the course of at least
24months of testosterone suppression. There may even be
small but signiﬁcant increases in BMD at the lumbar spine
[54, 55]. Some retrieved studies present data pertaining to
maintained BMD in transgender women after many years
of testosterone suppression. One such study concluded that
“BMD is preserved over a median of 12.5years” .
In support, no increase in fracture rates was observed
over 12months of testosterone suppression . Current
advice, including that from the International Society for
Clinical Densitometry, is that transgender women, in the
absence of other risk factors, do not require monitoring of
BMD [52, 57]. This is explicable under current standard
treatment regimes, given the established positive eﬀect
of estrogen, rather than testosterone, on bone turnover in
Given the maintenance of BMD and the lack of a plau-
sible biological mechanism by which testosterone sup-
pression might aﬀect skeletal measurements such as bone
length and hip width, we conclude that height and skeletal
parameters remain unaltered in transgender women, and
that sporting advantage conferred by skeletal size and bone
density would be retained despite testosterone reductions
compliant with the IOC’s current guidelines. This is of
particular relevance to sports where height, limb length
and handspan are key (e.g. basketball, volleyball, hand-
ball) and where high movement eﬃciency is advantageous.
Male bone geometry and density may also provide pro-
tection against some sport-related injuries—for example,
males have a lower incidence of knee injuries, often attrib-
uted to low quadriceps (Q) angle conferred by a narrow
pelvic girdle [59, 60].
4.2 Muscle andStrength Metrics
As discussed earlier, muscle mass and strength are key
parameters underpinning male performance advantages.
Strength diﬀerences range between 30 and 100%, depending
upon the cohort studied and the task used to assess strength.
Thus, given the important contribution made by strength to
performance, we sought studies that have assessed strength
and muscle/lean body mass changes in transgender women
after testosterone reduction. Studies retrieved in our litera-
ture search covered both longitudinal and cross-sectional
analyses. Given the superior power of the former study type,
we will focus on these.
The pioneer work by Gooren and colleagues, published
in part in 1999  and in full in 2004 , reported the
eﬀects of 1 and 3years of testosterone suppression and
estrogen supplementation in 19 transgender women (age
18–37years). After the first year of therapy, testoster-
one levels were reduced to 1nmol/L, well within typical
female reference ranges, and remained low throughout the
study course. As determined by MRI, thigh muscle area
had decreased by − 9% from baseline measurement. After
3years, thigh muscle area had decreased by a further − 3%
from baseline measurement (total loss of − 12% over 3years
of treatment). However, when compared with the baseline
measurement of thigh muscle area in transgender men (who
are born female and experience female puberty), transgender
women retained signiﬁcantly higher thigh muscle size. The
ﬁnal thigh muscle area, after three years of testosterone sup-
pression, was 13% larger in transwomen than in the transmen
at baseline (p < 0.05). The authors concluded that testos-
terone suppression in transgender women does not reverse
muscle size to female levels.
Including Gooren and Bunck , 12 longitudinal stud-
ies [53, 63–73] have examined the eﬀects of testosterone
suppression on lean body mass or muscle size in transgen-
der women. The collective evidence from these studies sug-
gests that 12months, which is the most commonly examined
intervention period, of testosterone suppression to female-
typical reference levels results in a modest (approximately
− 5%) loss of lean body mass or muscle size (Table4). No
206 E.N.Hilton, T.R.Lundberg
Table 4 Longitudinal studies of muscle and strength changes in adult transgender women undergoing cross-sex hormone therapy
Studies reporting measures of lean mass, muscle volume, muscle area or strength are included. Muscle/strength data are calculated in refer-
ence to baseline cohort data and, where reported, reference female (or transgender men before treatment) cohort data. Tack etal.  was not
included in the table since some of the participants had not completed full puberty at treatment initiation. van Caenegem etal.  reports refer-
ence female values measured in a separately-published, parallel cohort of transgender men
N number of participants, TW transgender women, Yr year, Mo month, T testosterone, E estrogen. ± Standard deviation (unless otherwise indi-
cated in text), LBM lean body mass, ALM appendicular lean mass
Study Participants (age) Therapy Conﬁrmed serum
testosterone levels Muscle/strength data Comparison with refer-
Polderman etal. N = 12 TW 18–36yr
(age range) T suppression + E
supplementation < 2nmol/L at 4 mo LBM
4 mo − 2.2% LBM
4 mo 16%
Gooren and Bunck
N = 19 TW 26 ± 6yr T suppression + E
supplementation ≤ 1nmol/L at 1 and
3yr Thigh area
1yr − 9% / 3yr -12% Thigh area
1yr 16%/3yr 13%
Haraldsen etal. N = 12 TW 29 ± 8yr E supplementation < 10nmol/L at 3 mo
and 1yr LBM
Mueller etal. N = 84 TW 36 ± 11yr T suppression + E
supplementation ≤ 1nmol/L at 1 and
1yr − 4%/2yr − 7%
Wierckx etal. N = 53 T W 31 ± 14yr T suppression + E
supplementation < 10nmol/L at 1yr LBM
1yr − 5% LBM
Van Caenegem etal.
(and Van Caenegem
N = 49 TW
33 ± 14yr T suppression + E
supplementation ≤ 1nmol/L at 1 and
1yr − 4%/2yr − 0.5%
1yr − 7%/2yr − 9%
1yr − 2%/2yr − 4%
1yr − 8%/2yr − 4%
1yr 24%/2yr 28%
1yr 26%/2yr 23%
1yr 16%/2yr 13%
1yr 29%/2yr 34%
Gava etal. N = 40 TW
31 ± 10yr T suppression + E
supplementation < 5nmol/L at 6 mo
and ≤ 1nmol/L at
1yr − 2%
Auer etal. N = 45 TW
35 ± 1 (SE) yr T suppression + E
supplementation < 5nmol/L at 1yr LBM
1yr − 3% LBM
Klaver etal. N = 179 TW
29 (range 18–66) T suppression + E
supplementation ≤ 1nmol/L at 1yr LBM 1yr
Total − 3%
Arm region − 6%
Trunk region − 2%
Android region 0%
Gynoid region − 3%
Leg region − 4%
Arm region 28%
Leg region 19%
Fighera etal. N = 46 TW
34 ± 10 E supplementation
with or without T
< 5nmol/L at 3 mo
≤ 1nmol/L at 31 mo ALM
31 mo − 4% from the
3 mo visit
Scharﬀ etal. N = 249 TW
28 (inter quartile
T suppression + E
supplementation ≤ 1nmol/L at 1yr Grip strength
1yr − 4% Grip strength
Wiik etal. N = 11 TW
27 ± 4 T suppression + E
supplementation ≤ 1nmol/L at 4 mo
and at 1yr Thigh volume
1yr − 5%
1yr − 4%
Knee ﬂexion strength
Knee extension strength
Knee ﬂexion strength
Eﬀects of Testosterone Suppression in Transgender Women
study has reported muscle loss exceeding the − 12% found
by Gooren and Bunck after 3years of therapy. Notably, stud-
ies have found very consistent changes in lean body mass
(using dual-energy X-ray absorptiometry) after 12months
of treatment, where the change has always been between
− 3 and − 5% on average, with slightly greater reductions in
the arm compared with the leg region . Thus, given the
large baseline diﬀerences in muscle mass between males
and females (Table1; approximately 40%), the reduction
achieved by 12months of testosterone suppression can rea-
sonably be assessed as small relative to the initial superior
mass. We, therefore, conclude that the muscle mass advan-
tage males possess over females, and the performance impli-
cations thereof, are not removed by the currently studied
durations (4months, 1, 2 and 3years) of testosterone sup-
pression in transgender women. In sports where muscle mass
is important for performance, inclusion is therefore only pos-
sible if a large imbalance in fairness, and potentially safety
in some sports, is to be tolerated.
To provide more detailed information on not only gross
body composition but also thigh muscle volume and con-
tractile density, Wiik etal.  recently carried out a com-
prehensive battery of MRI and computed tomography (CT)
examinations before and after 12months of successful tes-
tosterone suppression and estrogen supplementation in 11
transgender women. Thigh volume (both anterior and pos-
terior thigh) and quadriceps cross-sectional area decreased
− 4 and − 5%, respectively, after the 12-month period, sup-
porting previous results of modest eﬀects of testosterone
suppression on muscle mass (see Table4). The more novel
measure of radiological attenuation of the quadriceps mus-
cle, a valid proxy of contractile density [74, 75], showed no
signiﬁcant change in transgender women after 12months
of treatment, whereas the parallel group of transgender men
demonstrated a + 6% increase in contractile density with
As indicated earlier (e.g. Table1), the diﬀerence in mus-
cle strength between males and females is often more pro-
nounced than the diﬀerence in muscle mass. Unfortunately,
few studies have examined the eﬀects of testosterone sup-
pression on muscle strength or other proxies of performance
in transgender individuals. The ﬁrst such study was pub-
lished online approximately 1year prior to the release of
the current IOC policy. In this study, as well as reporting
changes in muscle size, van Caenegem etal.  reported
that hand-grip strength was reduced from baseline measure-
ments by − 7% and − 9% after 12 and 24months, respec-
tively, of cross-hormone treatment in transgender women.
Comparison with data in a separately-published, parallel
cohort of transgender men  demonstrated a retained
hand-grip strength advantage after 2years of 23% over
female baseline measurements (a calculated average of
baseline data obtained from control females and transgen-
In a recent multicenter study , examination of 249
transgender women revealed a decrease of − 4% in grip
strength after 12months of cross-hormone treatment, with
no variation between diﬀerent testosterone level, age or
BMI tertiles (all transgender women studied were within
female reference ranges for testosterone). Despite this mod-
est reduction in strength, transgender women retained a
17% grip strength advantage over transgender men meas-
ured at baseline. The authors noted that handgrip strength in
transgender women was in approximately the 25th percentile
for males but was over the 90th percentile for females, both
before and after hormone treatment. This emphasizes that
the strength advantage for males over females is inherently
large. In another study exploring handgrip strength, albeit
in late puberty adolescents, Tack etal. noted no change in
grip strength after hormonal treatment (average duration
11months) of 21 transgender girls .
Although grip strength provides an excellent proxy meas-
urement for general strength in a broad population, speciﬁc
assessment within diﬀerent muscle groups is more valu-
able in a sports-speciﬁc framework. Wiik etal.,  having
determined that thigh muscle mass reduces only modestly,
and that no signiﬁcant changes in contractile density occur
with 12months of testosterone suppression, provided, for
the ﬁrst time, data for isokinetic strength measurements of
both knee extension and knee ﬂexion. They reported that
muscle strength after 12months of testosterone suppression
was comparable to baseline strength. As a result, transgender
women remained about 50% stronger than both the group
of transgender men at baseline and a reference group of
females. The authors suggested that small neural learning
eﬀects during repeated testing may explain the apparent lack
of small reductions in strength that had been measured in
other studies .
These longitudinal data comprise a clear pattern of very
modest to negligible changes in muscle mass and strength
in transgender women suppressing testosterone for at least
12months. Muscle mass and strength are key physical
parameters that constitute a signiﬁcant, if not majority, por-
tion of the male performance advantage, most notably in
those sports where upper body strength, overall strength, and
muscle mass are crucial determinants of performance. Thus,
our analysis strongly suggests that the reduction in testoster-
one levels required by many sports federation transgender
policies is insuﬃcient to remove or reduce the male advan-
tage, in terms of muscle mass and strength, by any mean-
ingful degree. The relatively consistent ﬁnding of a minor
(approximately − 5%) muscle loss after the ﬁrst year of treat-
ment is also in line with studies on androgen-deprivation
therapy in males with prostate cancer, where the annual loss
208 E.N.Hilton, T.R.Lundberg
of lean body mass has been reported to range between − 2
and − 4% .
Although less powerful than longitudinal studies, we
identified one major cross-sectional study that meas-
ured muscle mass and strength in transgender women.
In this study, 23 transgender women and 46 healthy age-
and height-matched control males were compared .
The transgender women were recruited at least 3years
after sex reassignment surgery, and the mean duration of
cross-hormone treatment was 8years. The results showed
that transgender women had 17% less lean mass and 25%
lower peak quadriceps muscle strength than the control
males . This cross-sectional comparison suggests that
prolonged testosterone suppression, well beyond the time
period mandated by sports federations substantially reduces
muscle mass and strength in transgender women. However,
the typical gap in lean mass and strength between males and
females at baseline (Table1) exceeds the reductions reported
in this study . The ﬁnal average lean body mass of the
transgender women was 51.2kg, which puts them in the 90th
percentile for women . Similarly, the ﬁnal grip strength
was 41kg, 25% higher than the female reference value .
Collectively, this implies a retained physical advantage even
after 8years of testosterone suppression. Furthermore, given
that cohorts of transgender women often have slightly lower
baseline measurements of muscle and strength than control
males , and baseline measurements were unavailable for
the transgender women of this cohort, the above calculations
using control males reference values may be an overestimate
of actual loss of muscle mass and strength, emphasizing both
the need for caution when analyzing cross-sectional data in
the absence of baseline assessment and the superior power
of longitudinal studies quantifying within-subject changes.
4.3 Endurance Performance andCardiovascular
No controlled longitudinal study has explored the eﬀects of
testosterone suppression on endurance-based performance.
Sex diﬀerences in endurance performance are generally
smaller than for events relying more on muscle mass and
explosive strength. Using an age grading model designed
to normalize times for masters/veteran categories, Harper
 analyzed self-selected and self-reported race times for
eight transgender women runners of various age categories
who had, over an average 7year period (range 1–29years),
competed in sub-elite middle and long distance races within
both the male and female categories. The age-graded scores
for these eight runners were the same in both categories,
suggesting that cross-hormone treatment reduced running
performance by approximately the size of the typical male
advantage. However, factors aﬀecting performances in the
interim, including training and injury, were uncontrolled
for periods of years to decades and there were uncertainties
regarding which race times were self-reported vs. which race
times were actually reported and veriﬁed, and factors such as
standardization of race course and weather conditions were
unaccounted for. Furthermore, one runner improved sub-
stantially post-transition, which was attributed to improved
training . This demonstrates that performance decrease
after transition is not inevitable if training practices are
improved. Unfortunately, no study to date has followed up
these preliminary self-reports in a more controlled setting,
so it is impossible to make any ﬁrm conclusions from this
data set alone.
Circulating hemoglobin levels are androgen-dependent
 and typically reported as 12% higher in males compared
with females . Hemoglobin levels appear to decrease by
11–14% with cross-hormone therapy in transgender women
[62, 71], and indeed comparably sized reductions have
been reported in athletes with DSDs where those athletes
are sensitive to and been required to reduce testosterone
[47, 83]. Oxygen-carrying capacity in transgender women
is most likely reduced with testosterone suppression, with
a concomitant performance penalty estimated at 2–5% for
the female athletic population . Furthermore, there is a
robust relationship between hemoglobin mass and VO2max
[84, 85] and reduction in hemoglobin is generally associ-
ated with reduced aerobic capacity [86, 87]. However,
hemoglobin mass is not the only parameter contributing to
VO2max, where central factors such as total blood volume,
heart size and contractility, and peripheral factors such as
capillary supply and mitochondrial content also plays a role
in the ﬁnal oxygen uptake . Thus, while a reduction in
hemoglobin is strongly predicted to impact aerobic capacity
and reduce endurance performance in transgender women,
it is unlikely to completely close the baseline gap in aerobic
capacity between males and females.
The typical increase in body fat noted in transgender
women [89, 90] may also be a disadvantage for sporting
activities (e.g. running) where body weight (or fat distribu-
tion) presents a marginal disadvantage. Whether this body
composition change negatively aﬀects performance results
in transgender women endurance athletes remains unknown.
It is unclear to what extent the expected increase in body fat
could be oﬀset by nutritional and exercise countermeasures,
as individual variation is likely to be present. For example,
in the Wiik etal. study , 3 out of the 11 transgender
women were completely resistant to the marked increase in
total adipose tissue noted at the group level. This inter-indi-
vidual response to treatment represents yet another challenge
for sports governing bodies who most likely, given the many
obstacles with case-by-case assessments, will form policies
based on average eﬀect sizes.
Altogether, the effects of testosterone suppression
on performance markers for endurance athletes remain
Eﬀects of Testosterone Suppression in Transgender Women
insuﬃciently explored. While the negative eﬀect on hemo-
globin concentration is well documented, the eﬀects on
VO2max, left ventricular size, stroke volume, blood volume,
cardiac output lactate threshold, and exercise economy, all
of which are important determinants of endurance perfor-
mance, remain unknown. However, given the plausible dis-
advantages with testosterone suppression mentioned in this
section, together with the more marginal male advantage
in endurance-based sports, the balance between inclusion
and fairness is likely closer to equilibrium in weight-bearing
endurance-based sports compared with strength-based sports
where the male advantage is still substantial.
The data presented here demonstrate that superior anthro-
pometric, muscle mass and strength parameters achieved by
males at puberty, and underpinning a considerable portion
of the male performance advantage over females, are not
removed by the current regimen of testosterone suppression
permitting participation of transgender women in female
sports categories. Rather, it appears that the male perfor-
mance advantage remains substantial. Currently, there is no
consensus on an acceptable degree of residual advantage
held by transgender women that would be tolerable in the
female category of sport. There is signiﬁcant dispute over
this issue, especially since the physiological determinants
of performance vary across diﬀerent sporting disciplines.
However, given the IOC position that fair competition is
the overriding sporting objective , any residual advan-
tage carried by transgender women raises obvious concerns
about fair and safe competition in the numerous sports
where muscle mass, strength and power are key performance
5.1 Perspectives onAthletic Status ofTransgender
Whilst available evidence is strong and convincing that
strength, skeletal- and muscle-mass derived advantages will
largely remain after cross-hormone therapy in transgender
women, it is acknowledged that the ﬁndings presented here
are from healthy adults with regular or even low physical
activity levels , and not highly trained athletes.Thus, fur-
ther researchis required in athletic transgender populations.
However, despite the current absence of empirical evi-
dence in athletic transgender women, it is possible to
evaluate potential outcomes in athletic transgender women
compared with untrained cohorts.The first possibility
is thatathletic transgender women will experience simi-
lar reductions (approximately − 5%) in muscle mass and
strength as untrained transgender women, and will thus
retain signiﬁcant advantages over a comparison group of
females. As a result of higher baseline characteristics in
these variables, the retained advantage may indeed be even
larger. A second possibility is that by virtue of greater mus-
cle mass and strength at baseline, pre-trained transgender
women will experience larger relative decreases in muscle
mass and strength if they converge with untrained transgen-
der women, particularly if training is halted during transi-
tion. Finally, training before and during the period of testos-
terone suppression may attenuate the anticipated reductions,
such that relative decreases in muscle mass and strength
will be smaller or non-existent in transgender women who
undergo training, compared to untrained (and non-training)
It is well established that resistance training counteracts
substantial muscle loss during atrophy conditions that are
far more severe than testosterone suppression. For exam-
ple, resistance exercise every third day during 90-days bed
rest was suﬃcient to completely oﬀset the 20% reduction
in knee extensor muscle size noted in the resting control
subjects . More relevant to the question of transgender
women, however, is to examine training eﬀects in studies
where testosterone has been suppressed in biological males.
Kvorning etal. investigated, in a randomized placebo-con-
trolled trial, how suppression of endogenous testosterone
for 12weeks inﬂuenced muscle hypertrophy and strength
gains during a training program (3days/week) that took
place during the last 8weeks of the 3-month suppression
period. Despite testosterone suppression to female lev-
els of 2nmol/L, there was a signiﬁcant + 4% increase in leg
lean mass and a + 2% increase in total lean body mass,and
a measurable though insigniﬁcant increase in isometric knee
extension strength.Moreover, in select exercises used dur-
ing the training program, 10RM leg press and bench press
increased + 32% and + 17%, respectively. While some of the
training adaptations were lower than in the placebo group,
this study demonstrates that training during a period of tes-
tosterone suppression not only counteracts muscle loss, but
can actually increase muscle mass and strength.
Males with prostate cancer undergoing androgen depri-
vation therapy provide a second avenue to examine train-
ing eﬀects during testosterone suppression. Testosterone
levels are typically reduced to castrate levels, and the loss
of lean masshas typically rangedbetween − 2 and − 4% per
year, consistent with the ﬁndings described previously
in transgender women. A recent meta-analysis concluded
that exercise interventions including resistance exercise
were generally eﬀective for maintaining muscle mass and
increasing muscle strength in prostate cancer patients under-
going androgen deprivation therapy. It is important to
emphasize that the eﬃcacy of the diﬀerent training programs
may vary. For example, a 12-week training study of prostate
cancer patients undergoing androgen deprivation therapy
210 E.N.Hilton, T.R.Lundberg
included drop-sets to combine heavy loads and high volume
while eliciting near-maximal eﬀorts in each set. This
strategy resulted in signiﬁcantly increased lean body mass
(+ 3%), thigh muscle volume (+ 6%), knee extensor 1RM
strength (+ 28%) and leg press muscle endurance (+ 110%).
In addition to the described eﬀects of training during tes-
tosterone suppression, the eﬀect of training prior to testos-
terone suppression may also contribute to the attenuation
of any muscle mass and strength losses, via a molecular
mechanism referred to as ‘muscle memory’ . Speciﬁ-
cally, it has been suggested that myonuclei acquired by
skeletal muscle cells during training are maintained during
subsequent atrophy conditions . Even though this model
of muscle memory has been challenged recently , it may
facilitate an improved training response upon retraining .
Mechanistically, the negative eﬀects of testosterone suppres-
sion on muscle mass are likely related to reduced levels of
resting protein synthesis, which, together with protein
breakdown, determines the net protein balance of skeletal
muscle. However, testosterone may not be required to elicit a
robust muscle protein synthesis response to resistance exer-
cise. Indeed, relative increases in muscle mass in men
and women from resistance training are comparable, despite
marked diﬀerences in testosterone levels, and the acute
rise in testosterone apparent during resistance exercise does
not predict muscle hypertrophy nor strength gains.
This suggests that even though testosterone is important for
muscle mass, especially during puberty, the maintenance
of muscle mass through resistance training is not crucially
dependent on circulating testosterone levels.
Thus,in well-controlled studies in biological males who
train while undergoing testosterone reduction, training
is protective of, and may even enhance, muscle mass and
strength attributes. Considering transgender women ath-
letes who train during testosterone suppression, it is plau-
sible to conclude that any losses will be similar to or even
smaller in magnitude than documented in the longitudinal
studies described in this review. Furthermore, pre-trained
transgender women are likely to have greater muscle mass
at baseline than untrained transgender women; it is possi-
ble that even with the same, rather than smaller, relative
decreases in muscle mass and strength, the magnitude of
retained advantage will be greater. In contrast, if pre-trained
transgender women undergo testosterone suppression while
refraining from intense training, it appears likely that muscle
mass and strength will be lost at either the same or greater
rate than untrained individuals, although there is no rationale
to expect a weaker endpoint state. The degree of change in
athletic transgender women is inﬂuenced by the athlete’s
baseline resistance-training status, the eﬃcacy of the imple-
mented program and other factors such as genetic make-up
and nutritional habits, but we argue that it is implausible that
athletic transgender women would achieve ﬁnal muscle mass
and strength metrics that are on par with reference females
at comparable athletic level.
5.2 The Focus onMuscle Mass andStrength
We acknowledge that changes in muscle mass are not always
correlated in magnitude to changes in strength measure-
ments because muscle mass (or total mass) is not the only
contributor to strength . Indeed, the importance of the
nervous system, e.g. muscle agonist activation (recruitment
and ﬁring frequency) and antagonist co-activation, for mus-
cle strength must be acknowledged . In addition, factors
such as ﬁber types, biomechanical levers, pennation angle,
fascicle length and tendon/extracellular matrix composition
may all inﬂuence the ability to develop muscular force .
While there is currently limited to no information on how
these factors are inﬂuenced by testosterone suppression, the
impact seems to be minute, given the modest changes noted
in muscle strength during cross-hormone treatment.
It is possible that estrogen replacement may aﬀect the
sensitivity of muscle to anabolic signaling and have a pro-
tective eﬀect on muscle mass  explaining, in part, the
modest change in muscle mass with testosterone suppression
and accompanying cross-hormone treatment. Indeed, this is
supported by research conducted on estrogen replacement
therapy in other targeted populations [107, 108] and in sev-
eral diﬀerent animal models, including mice after gonadec-
tomy  and ovariectomy .
In terms of other performance proxies relevant to sports
performance, there is no research evaluating the eﬀects of
transgender hormone treatment on factors such as agility,
jumping or sprint performance, competition strength perfor-
mance (e.g. bench press), or discipline-speciﬁc performance.
Other factors that may impact sports performance, known
to be aﬀected by testosterone and some of them measurably
diﬀerent between males and females, include visuospatial
abilities, aggressiveness, coordination and ﬂexibility.
5.3 Testosterone‑Based Criteria forInclusion
ofTransgender Women inFemale Sports
The appropriate testosterone limit for participation of
transgender women in the female category has been a matter
of debate recently, where sports federations such as World
Athletics recently lowered the eligibility criterion of free
circulating testosterone (measured by means of liquid chro-
matography coupled with mass spectrometry) to < 5nmol/L.
This was based, at least in part, on a thorough review by
Handelsman etal. , where the authors concluded that,
given the nonoverlapping distribution of circulating testos-
terone between males and females, and making an allowance
Eﬀects of Testosterone Suppression in Transgender Women
for females with mild hyperandrogenism (e.g. with poly-
cystic ovary syndrome), the appropriate testosterone limit
should be 5 rather than 10nmol/L.
From the longitudinal muscle mass/strength studies sum-
marised here, however, it is apparent that most therapeutic
interventions result in almost complete suppression of tes-
tosterone levels, certainly well below 5nmol/L (Table4).
Thus, with regard to transgender women athletes, we ques-
tion whether current circulating testosterone level cut-oﬀ
can be a meaningful decisive factor, when in fact not even
suppression down to around 1nmol/L removes the anthro-
pometric and muscle mass/strength advantage in any sig-
In terms of duration of testosterone suppression, it may
be argued that although 12months of treatment is not
suﬃcient to remove the male advantage, perhaps extend-
ing the time frame of suppression would generate greater
parity with female metrics. However, based on the studies
reviewed here, evidence is lacking that this would diminish
the male advantage to a tolerable degree. On the contrary,
it appears that the net loss of lean mass and grip strength is
not substantially decreased at year 2 or 3 of cross-hormone
treatment (Table4), nor evident in cohorts after an average
8years after transition. This indicates that a plateau or a new
steady state is reached within the ﬁrst or second year of treat-
ment, a phenomenon also noted in transgender men, where
the increase in muscle mass seems to stabilise between the
ﬁrst and the second year of testosterone treatment .
We have shown that under testosterone suppression
regimes typically used in clinical settings, and which
comfortably exceed the requirements of sports federations
for inclusion of transgender women in female sports cat-
egories by reducing testosterone levels to well below the
upper tolerated limit, evidence for loss of the male perfor-
mance advantage, established by testosterone at puberty
and translating in elite athletes to a 10–50% performance
advantage, is lacking. Rather, the data show that strength,
lean body mass, muscle size and bone density are only
trivially aﬀected. The reductions observed in muscle mass,
size, and strength are very small compared to the baseline
diﬀerences between males and females in these variables,
and thus, there are major performance and safety implica-
tions in sports where these attributes are competitively sig-
niﬁcant. These data signiﬁcantly undermine the delivery
of fairness and safety presumed by the criteria set out in
transgender inclusion policies, particularly given the stated
prioritization of fairness as an overriding objective (for the
IOC). If those policies are intended to preserve fairness,
inclusion and the safety of biologically female athletes,
sportingorganizations may need to reassess their policies
regarding inclusion of transgender women.
From a medical-ethical point of view, it may be ques-
tioned as to whether a requirement to lower testosterone
below a certain level to ensure sporting participation
can be justiﬁed at all. If the advantage persists to a large
degree, as evidence suggests, then a stated objective of
targeting a certain testosterone level to be eligible will not
achieve its objective and may drive medical practice that
an individual may not want or require, without achieving
its intended beneﬁt.
The research conducted so far has studied untrained
transgender women. Thus, while this research is impor-
tant to understand the isolated effects of testosterone
suppression, it is still uncertain how transgender women
athletes, perhaps undergoing advanced training regimens
to counteract the muscle loss during the therapy, would
respond. It is also important to recognize that performance
in most sports may be inﬂuenced by factors outside mus-
cle mass and strength, and the balance between inclu-
sion, safety and fairness therefore diﬀers between sports.
While there is certainly a need for more focused research
on this topic, including more comprehensive performance
tests in transgender women athletes and studies on train-
ing capacity of transgender women undergoing hormone
therapy, it is still important to recognize that the biological
factors underpinning athletic performance are unequivo-
cally established. It is, therefore, possible to make strong
inferences and discuss potential performance implications
despite the lack of direct sport-speciﬁc studies in athletes.
Finally, since athlete safety could arguably be described
as the immediate priority above considerations of fairness
and inclusion, proper risk assessment should be conducted
within respective sports that continue to include transgen-
der women in the female category.
If transgender women are restricted within or excluded
from the female category of sport, the important question
is whether or not this exclusion (or conditional exclusion)
is necessary and proportionate to the goal of ensuring fair,
safe and meaningful competition. Regardless of what the
future will bring in terms of revised transgender policies, it
is clear that diﬀerent sports diﬀer vastly in terms of physi-
ological determinants of success, which may create safety
considerations and may alter the importance of retained
performance advantages. Thus, we argue against universal
guidelines for transgender athletes in sport and instead
propose that each individual sports federation evaluate
their own conditions for inclusivity, fairness and safety.
Compliance with Ethical Standards
Funding None. Open access funding provided by Karolinska Institutet.
212 E.N.Hilton, T.R.Lundberg
Conflicts of interest Emma N Hilton and Tommy R Lundberg declare
that they have no conﬂict of interest with the content of this review.
Authorship contributions Both authors (ENH and TRL) were involved
in the conception and design of this paper, and both authors drafted,
revised and approved the ﬁnal version of the paper.
Ethics approval Not applicable.
Informed consent Not applicable.
Data availability Available upon request.
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