Youth-Specific Athletic Performance Data After Gender-Affirming Hormone Therapy

What the evidence says

None of the claims here report a measured athletic-performance outcome — a race time, a competition result, a sport-specific test — in a youth cohort. What exists instead is physiological proxy data: body composition, regional muscle cross-sectional area (CSA), strength, and hemoglobin, all measured during gender-affirming hormone therapy (GAHT) in adolescents. That is an absence worth stating plainly: this page cannot speak to youth athletic performance directly, only to the physiological changes performance is often assumed to track.

The best-supported finding concerns body composition. A prospective cohort study (Wolf 2025, tier A) reports that adolescent transgender women decrease lean mass during the first year of oestrogen treatment; total fat mass then stays stable or increases over the following period, but remains lower than in cisgender women after three years. The same study reports the mirror pattern in adolescent transgender men on testosterone: lean body mass increases and total fat mass decreases in year 1, these changes largely hold through three years, and total fat mass becomes comparable to cisgender males by that point.

A separate source narrows in on muscle size using imaging. Harper 2021 (tier B) reports that in one study of adolescent transgender women whose testosterone reached only 8.8 nmol/L after treatment, forearm cross-sectional area decreased 4.1% and calf cross-sectional area decreased 8.9% over 12 months. This points the same direction as the whole-body lean mass decrease Wolf reports, though the two are different measures from different studies.

Two other outcomes in youth-aged cohorts showed no measured effect. Strength did not change over 12 months in one adolescent study — the same 8.8 nmol/L cohort as the CSA finding. Hemoglobin did not change over 6 months in a youth cohort with mean age 18 (range 14–25) whose testosterone reached only 6.9 nmol/L. Both null results come from cohorts the source describes as not reaching typical female testosterone levels, which it flags as a possible explanation for the lack of measured change.

Where the evidence splits

The clearest internal split sits inside a single cohort. The same adolescent group (final testosterone 8.8 nmol/L) shows decreased muscle cross-sectional area (forearm −4.1%, calf −8.9% over 12 months) but no measured change in strength over the same 12-month window. Muscle size went down; measured strength did not follow it down. The source offers "suboptimal testosterone suppression and small adolescent sample" as a possible explanation for the null strength result, but that same suppression level is the one under which the CSA decrease was measured — so why smaller muscles didn't yield a measured strength change is unexplained by the sources on this page.

Thresholds and dose-response

No threshold established in the current sources. Two claims report testosterone levels associated with null results — 8.8 nmol/L (strength) and 6.9 nmol/L (hemoglobin) — and both are described as falling short of "typical female testosterone levels," but no source states what that reference level is or establishes a specific level at which either outcome starts, stops, or reverses.

Open empirical questions

• No claim here measures actual athletic performance (times, scores, competition outcomes) in youth cohorts — only physiological proxies.
• What testosterone level constitutes "typical female range" for suppression, and whether reaching it changes the null strength and hemoglobin results, is not measured here.
• Why measured muscle CSA decreased in the 8.8 nmol/L cohort while measured strength did not is not explained by the sources on this page.
• No data here indicate how long body composition changes take to stabilize beyond the 1-year and 3-year checkpoints Wolf reports.