Gender-affirming hormone therapy (GAHT) is a fundamental component of medical transition in transgender men, promoting body composition changes that align physical characteristics with gender identity and alleviate gender dysphoria. In adults, GAHT typically involves testosterone administration, whereas adolescents may receive gonadotropin-releasing hormone agonists to suppress puberty before initiating testosterone.

Despite its general safety when appropriately monitored, findings on GAHT-related changes in body composition and potential cardiovascular implications are inconsistent. Accurate assessment of skeletal muscle mass and fat redistribution is clinically relevant, as conventional anthropometric measures may fail to capture these changes.

This study evaluates body composition changes after one year of testosterone therapy in transgender men using bioelectrical impedance vector analysis (BIVA), and explores the utility of muscle ultrasound as an accessible tool for monitoring skeletal muscle and potential differences among testosterone formulations.

Gender-affirming hormone therapy (GAHT) is a central component of medical transition in transgender men, and aims to induce body composition changes that align physical appearance with gender identity and reduce gender dysphoria. In adults, GAHT typically consists of testosterone administration, whereas, in adolescents, gonadotropin-releasing hormone agonists (GnRHa) may be used to suppress puberty before initiating testosterone. GAHT has been associated with improvements in psychological well-being, social integration, and quality of life, although evidence remains limited due to the lack of randomized clinical trials. Moreover, body composition changes have been linked to psychological health in this population.

GAHT is considered safe when adequately monitored and individualized according to cardiovascular risk profile and pre-existing conditions. However, current evidence regarding its effects on body composition and potential implications for cardiovascular risk is inconsistent. Clinically, it is particularly relevant to determine the extent to which GAHT supports gains in skeletal muscle mass and fat redistribution, given their influence on body satisfaction and cardiometabolic risk. Importantly, relying on basic anthropometric measures such as body weight may lead to an underestimation of cardiovascular risk by failing to capture changes in key body compartments.

Reference techniques such as DXA or magnetic resonance imaging have been used to characterize these changes, although their limited accessibility restricts their routine application in clinical follow-up. In this context, bioelectrical impedance vector analysis (BIVA) and muscle ultrasound emerge as accessible alternatives for assessing skeletal muscle mass. However, the lack of population-specific reference values may hinder interpretation, underscoring the need to develop dedicated standards and validate these methods in transgender populations.

To examine the effect of one year of testosterone treatment on body composition in transgender men, different parameters will be assessed using BIVA. Additionally, the utility of muscle ultrasound as a feasible tool for monitoring skeletal muscle during masculinization will be explored, as well as if there are differences in body composition between different types of testosterone formulations.

Inclusion Criteria:

• Transgender men with confirmed gender dysphoria according to DSM-V criteria by an experienced sexologist.
• Testosterone-naïve status
• ≥14 years
• Absence of prior or planned mastectomy or genital surgery during the study period

Exclusion Criteria:

• Diagnosed eating disorders, severe illness, neuromuscular or malignant disease, cardiovascular disease, and/or diabetes mellitus
• Conditions contraindicating bioelectrical impedance analysis (pregnancy, lactation, or pacemaker)
• Started GAHT prior to inclusion.

Participants included individuals who identified as men, as well as non-binary individuals seeking or identifying with a masculine physical appearance.

Participants were recruited from the Gender Identity Unit of the Department of Endocrinology and Nutrition at University Hospital Doctor Peset (Valencia, Spain). All individuals underwent psychological evaluation by an experienced sexologist, who confirmed gender dysphoria according to DSM-V criteria, and prior to inclusion had been approved for gender-affirming medical intervention.