Randomized Controlled Trial Evidence for Puberty Blockers or Gender-Affirming Hormones

What the evidence says

None of the sources collected here describe a randomized controlled trial of puberty blockers or gender-affirming hormone therapy (GAHT). Every claim comes from observational designs: systematic reviews of observational studies, pre-post cohorts, and retrospective cohorts. The foundational puberty blocker studies that much of the later literature builds on had no control group: they started with 196 patients, reported 70 in the first study and 55 in the second, and had complete data for only 40, with 3 patients excluded after developing diabetes or morbid obesity and one death from postsurgical necrotizing fasciitis. Concurrent psychiatric support for all patients is noted as a possible confounder relative to current practice, and no physical health outcomes were assessed in that cohort.

On metabolic and skeletal outcomes, sex steroid therapy in transgender males is associated with increased serum triglycerides and LDL cholesterol and decreased HDL cholesterol across all follow-up periods; in transgender females, only triglycerides increase, with no measured change in other lipid parameters. On bone, transgender females show increased lumbar spine bone mineral density (BMD) at 12 and 24 months compared with baseline before feminizing hormone therapy, while transgender males show no measured difference in lumbar spine, femoral neck, or total hip BMD at the same time points. A systematic review and meta-analysis found transgender people at higher cardiovascular disease risk than cisgender people of the same birth sex, including increased risk of stroke, myocardial infarction, and venous thromboembolism, with high certainty evidence specifically for cardiovascular events in transmasculine individuals under 26 treated with testosterone.

On mental health, observational studies are described as consistently finding neutral or beneficial outcomes associated with receipt of gender-affirming care, without a quantified magnitude. Separately, pre-post studies of GAHT showed psychological improvement at 12-month follow-up, but this evidence is rated low certainty because concurrent psychological support and psychotropic medications confound the estimate. Other systematic and literature reviews found the evidence that GAHT improves mental health or reduces suicidality to be low or very low certainty, with small effect sizes of potentially little clinical significance. On sexual function, 67% of sexually active participants in a transfeminine cohort that received puberty blockers and vaginoplasty (n=37) regularly experienced one or more sexual difficulties; in a separate cohort of adults who began puberty suppression before age 18, 58% of transmasculine (n=50) and 50% of transfeminine (n=20) participants reported sexual dysfunction, though no inferential statistics were run and the samples are described as likely underpowered.

For puberty blockers specifically, the systematic review commissioned by the Cass Review could not draw reliable conclusions on gender-related outcomes, psychological and psychosocial health, cognitive development, or fertility; it did find that bone health and height might be compromised. The only study that followed patients from puberty blockers through GAHT into adulthood — the original Dutch cohort — did not examine physical health outcomes. A separate review found very low certainty evidence for both bone mineral density and sexual dysfunction with GAHT in people under 26.

Outside outcome measurement, the physiological timeline of hormone therapy is documented directly: hormonal suppression is reached around 4 weeks after initiation, the first visible changes in secondary sex characteristics appear between 1 and 6 months, and other desired changes may not appear for up to a year.

Where the evidence splits

Mental health certainty is contested. Observational studies are reported as consistently showing neutral or beneficial mental health outcomes associated with gender-affirming care receipt, and pre-post GAHT studies showed psychological improvement at 12 months. But other systematic and literature reviews rate the evidence for mental health improvement or reduced suicidality as low or very low certainty, with small effect sizes of potentially little clinical significance, and the 12-month pre-post improvement is itself flagged as confounded by concurrent psychological support and psychotropic medication use. The direction of effect is not disputed across these sources — the disagreement is over how much confidence the underlying study designs support, and the claims attribute at least part of that gap to confounding from concurrent interventions rather than to conflicting results.

Thresholds and dose-response

No threshold established in the current sources.

Open empirical questions

• No randomized controlled trial evidence appears in these sources for puberty blockers or GAHT on any outcome — gender-related outcomes, psychological health, cognitive development, and fertility are explicitly flagged as unresolved by the Cass Review's systematic review.
• Physical health outcomes for people who moved from puberty blockers through GAHT into adulthood remain unmeasured — the only cohort that followed this pathway (de Vries et al. 2014) did not assess it.
• Whether the reported sexual dysfunction rates (67%, 58%, 50%) are caused by puberty blockers specifically, versus other factors, is unresolved — the largest cohort ran no inferential statistics.
• Fracture rates associated with the measured bone mineral density changes are not established — the source notes minimal information on fracture rates despite reporting BMD changes.