High-performance sports aim to achieve peak athletic performance in elite competitions. Sports where success depends on strength, speed, or endurance are conventionally divided into male and female categories to promote perceived fairness in competition.1 However, this binary classification based on male and female categories does not account for transgender individuals who experience incongruence between their biological sex and gender identity.2

Historically, cisgender women (CW) have faced numerous obstacles in participating in competitive sports, such as family care responsibilities and lower funding, among others. This has resulted in the underrepresentation of CW in high-level championships3: at the 1928 Olympics, only 9.6% of participants were women (277 athletes).4 With advances in gender equality, the role of women in sports has grown significantly, with 47.8% female participation at the 2021 Tokyo Olympics (5,457 athletes), culminating in a record-breaking participation at the 2024 Paris Olympics, achieving gender parity.5 In Spain, regulations such as the European Charter for Sport for All, adopted by the Council of Europe in 1975, have contributed to promoting equal opportunities for women, serving as a framework to combat gender-based discrimination in sports.

The inclusion of transgender athletes in sports has undergone multiple changes. Initially, sports regulations did not specifically address the participation of transgender athletes, creating challenges and barriers to their inclusion in competitions. On the other hand, some have warned that the indiscriminate inclusion of transgender women (TW) in elite sports could disrupt the structure and organization of sports by undermining principles of fairness. Organizations such as the International Olympic Committee (IOC) have played a crucial role in evolving regulations for transgender athlete participation. These guidelines have shifted over time, from strict requirements such as gonadectomy to criteria based on total testosterone (TT) levels and duration of gender-affirming hormone therapy (GAHT),4, 6

In 2003, the IOC proposed legal recognition of acquired gender, gonadectomy, and anti-androgenic treatment for at least 2 years. Afterwards, in 2015, the IOC required a maintained gender identity for at least 4 years and TT levels < 10 nmol/L for at least 1 year. In 2021, the IOC published the “Framework on Fairness, Inclusion, and Non-Discrimination on the Basis of Gender Identity and Sex Variations”,7 abandoning the need for anti-androgenic treatment, prioritizing the inclusion of all groups in sports,8 and delegating the regulation of transgender athlete participation to international federations (IFs). This regulation should also be nuanced according to the European Court of Human Rights (ECHR), which recognizes fairness in competitions as a legitimate and fundamental objective of competitive sports, allowing restrictions on individual rights to ensure fairness with a fair balance between competing interests (ECHR ruling, January 18th, 2018). Currently, some federations, such as Athletics9 and the World Boxing Council,10 have opted to exclude TW from female competitions to maintain fairness and safety. Others, like the Baseball Federation, allow their participation under the condition that TT levels remain below 10 nmol/L and medical statements from endocrinologists are provided. However, some federations, such as Skiing, have yet to establish specific regulations.

Sex-based categorization in elite sports is based on the discrepancy in androgen levels after puberty, particularly TT, which has significant implications for the cardiovascular system and musculoskeletal structure. On average, cisgender men (CM) have greater height, bone mineral density (BMD), muscle strength, hemoglobin concentration, and aerobic capacity (absolute and relative VO2 max) vs. CW. Performance differences vary by sport. On average, CM outperform CW by 10–12% in rowing, swimming, and running, 20% in jumping, and over 50% in throwing events (e.g., baseball).11 In TW, post-pubertal pharmacological suppression of TT levels for > 2 years leads to changes in body composition, including increased fat mass (FM) and decreased muscle mass and strength. However, other factors such as height, wingspan, and cardiorespiratory size remain unchanged.11 In cisgender populations, the differences between men and women in various sports are well-documented. In contrast, there is a lack of high-quality scientific literature evaluating the impact of GAHT or surgical treatment on physical performance in TW vs. CW.

The objectives of this literature review are to understand the current evidence on the effects of GAHT in TW on various factors impacting athletic performance: analytical parameters, body composition, BMD, muscle strength and area, and aerobic capacity; to assess the differences between TW and CW; and to evaluate the implications for elite sports competitions.

Following a critical and comprehensive literature review, members of the Endocrinology, Nutrition, and Physical Exercise Working Group (GT-GENEFSEEN) and the Gonad, Identity, and Sexual Differentiation Working Group (GT-GIDSEEN) of the Spanish Society of Endocrinology and Nutrition (SEEN) met to analyze the physiological differences between CW and TW in sports. In planning this meeting, the coordinators requested volunteers from both groups based on availability and interest in the topic, selecting a total of 6 members, all of whom were medical specialists in Endocrinology and Nutrition. Additionally, the director of the Women and Sports Seminar at Universidad Politécnica de Madrid (Madrid, Spain) participated in the article review.