What Did the Cass Review Find?
Key Takeaways
The Cass Review is an independent review of National Health Service (NHS) gender identity services for young people in England, led by Dr. Hilary Cass and published in April 2024.
Its central finding concerns evidence certainty, not harm: much of the existing research on psychological and hormone-related outcomes was rated low or very low quality.
The Endocrine Society responded in May 2024, saying the review adds no research contradicting its own guideline, and still calls monitored hormone therapy evidence-based and medically necessary.
The review's methodology has drawn its own criticism. The World Professional Association for Transgender Health (WPATH) and the United States Professional Association for Transgender Health (USPATH) said its conclusions relied on selective use of evidence.
The Cass Review has no legal authority in the United States, but it appears in the 2025 U.S. Department of Health and Human Services (HHS) report and in litigation over youth care.
The short answer
The Cass Review is an independent review of NHS gender identity services for young people in England, led by Dr. Hilary Cass and published in April 2024. Its central finding was that much of the research behind psychological support, puberty-pausing medication, and hormone treatment for gender dysphoria in young people is of low or very low certainty, a finding about the strength of the data, not proof the care causes harm. At TransHealthHub, we lay out what the review found, how U.S. medical societies responded, and where its methodology has been challenged.
What the Cass Review actually is
The Cass Review is a government-commissioned review of one national health system's pediatric gender services, not a study of U.S. clinics or law. NHS England commissioned it to make recommendations on NHS gender identity services for young people questioning their gender identity or experiencing gender dysphoria. Dr. Hilary Cass, a pediatrician, led the review, drawing on multiple systematic evidence reviews, and published its final report in April 2024.
What the review found about the evidence
The review's headline finding concerns evidence quality, not demonstrated harm. Its final report states that "systematic evidence reviews demonstrated the poor quality of the published studies, meaning there is not a reliable evidence base upon which to make clinical decisions." On puberty-pausing medication, it says "the rationale for early puberty suppression remains unclear, with weak evidence regarding the impact on gender dysphoria, mental or psychosocial health," calling long-term cognitive and psychosexual effects unknown. On hormone treatment, it says use under 18 "presents many unknowns" despite decades of adult use. A low-certainty rating means confidence in the results is limited, not that a treatment causes harm.
How U.S. medical societies have responded
The American Medical Association, American Academy of Pediatrics, American Psychological Association, and Endocrine Society continue to describe this care, when monitored appropriately, as evidence-based and medically necessary, positions held before the review and maintained since. The Endocrine Society addressed it in a May 8, 2024 statement, saying "NHS England's recent report, the Cass Review, does not contain any new research that would contradict the recommendations made in our Clinical Practice Guideline on gender-affirming care," a guideline it says cites more than 260 studies and recommends a conservative, staged approach.
The methodological critique
The review's methodology is contested among professional bodies, not just advocacy groups. WPATH and USPATH issued a joint response in May 2024 arguing the Cass Review "relies on selective and inconsistent use of evidence" and that its conclusions were "not supported by a robust methodology." Separately, a 2025 New England Journal of Medicine (NEJM) Perspective by Daniel Aaron and Craig Konnoth examined the review through a U.S. law-and-policy lens, arguing that how its authorship and contributors were handled would raise procedural concerns under U.S. research-transparency norms, a legal argument, not a clinical finding.
How the review fits into the U.S. evidence conversation
The Cass Review carries no legal weight in the United States, but it is a fixture of the U.S. evidence debate. The 2025 HHS report cites it extensively when assessing evidence quality, and KFF's neutral analysis notes a similar conclusion: "the quality of evidence on the effects of any intervention is low, and evidence on harms is 'sparse.'" KFF also flags a difference: the Cass Review states no LGBTQ+ group should face conversion practice, while the HHS report treats exploratory talk therapy more favorably. The review has also surfaced in litigation over youth care.
What the Cass Review found | How U.S. medical societies responded |
Psychological and hormone-related outcome studies rated low or very low certainty | Endocrine Society: no new research contradicts its guideline, which cites 260-plus studies |
Rationale for early puberty suppression called unclear | Guideline already recommends a conservative, staged approach |
Long-term hormone effects under 18 called largely unknown | Societies continue calling monitored hormone therapy evidence-based and medically necessary |
Recommended more research and caution within NHS services | WPATH and USPATH say conclusions did not consistently follow the evidence |
Frequently asked questions
Did the Cass Review find that this care is harmful? No. Its central finding was that much of the existing research is of low or very low certainty, a statement about the data, not proof of harm.
Does the Cass Review apply to U.S. health care? No. It reviewed NHS services in England only, though it is often cited in U.S. policy and legal debates.
Why do U.S. medical societies disagree with some conclusions? WPATH and USPATH say the review used evidence selectively, while the Endocrine Society says it added no research that changes its own guideline.
Further Reading and Resources
This page addresses a health topic and reflects the evidence as of September 2026. It is not medical advice. Decisions about a young person's care belong to that young person, their family, and their clinicians.
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