Home › Research › Cass on hormones

Cass Review on cross-sex hormones: irreversible damage without evidence

Cross-sex hormones — testosterone in girls, estrogen plus anti-androgens in boys — cause largely irreversible physical changes. The Cass Review delivers a scathing verdict: 99 percent of the studies are of low or very low quality. Hormones are prescribed to minors on an evidence basis that would not be accepted in any other branch of medicine. Transition does not cure — it confirms a misdiagnosis and causes permanent physical damage.

What cross-sex hormones do — and what cannot be reversed

In girls, testosterone leads to voice deepening, beard growth, clitoral enlargement, loss of pubic hair, fat redistribution, and infertility risks. In boys, estrogen plus anti-androgens leads to breast development, fat redistribution, testicular atrophy, and presumably permanent infertility. Voice deepening, beard growth, and remnants of mammoplasty are irreversible; "trans men" and "trans women" who regret their decision are left with these consequences for the rest of their lives. Anyone who presents these procedures as "ordinary medicine" is lying about the nature of the treatment.

What does the Cass Review say?

The York Systematic Review of Cross-Sex Hormones found that 99 percent of the studies fell under low or very low GRADE quality. Specifically:

• No conclusive evidence that hormones improve psychological well-being in the long term.
• No conclusive evidence that hormones prevent suicide — the "transition or suicide" rhetoric lacks an evidence base.
• Known risks: cardiovascular problems, infertility, bone density loss, cancer risk (long-term unknown).
• Informed consent for adolescents is effectively impossible for decisions with lifelong consequences.

The problem with the affirmative "studies"

The studies cited by the affirmative field — Tordoff, Olson-Kennedy, De Vries — have high dropout rates, short follow-up, no control group, or selective outcome measures. The Cass Review explicitly mentions this. De Vries' Dutch cohort is still cited everywhere, but the original 70 carefully selected adolescents have never been equaled — see VUmc protocol and publication bias . Olson-Kennedy even withheld outcomes that "did not please." That is not science; that is advocacy.

International conclusion: four Scandinavian countries plus the UK withdraw hormones

The NHS is drastically scaling back hormone administration for young people under 18. The Swedish Karolinska ( SBU 2022 ) restricts hormones to research settings. Finland's COHERE guideline does the same. Norway's Ukom report labels hormones for minors as experimental. Denmark followed with stricter policies in 2023. Four European countries plus the UK — independently of each other, all four based on GRADE evidence review — reach one conclusion: this should not be routine for children.

Detransitioners are no longer silent

The group of people who have used hormones and regret it is growing. Their testimonies—documented in detransition research and regret research —show that healthcare providers presented the treatment as "safe and reversible," while that is demonstrably untrue. For a long time, they were silenced by the affirmative lobby. That is no longer possible.