The Cass Review Explained

The Cass Review is an independent investigation into paediatric gender care in England. It was commissioned by NHS England in autumn 2020, led by paediatrician Dr Hilary Cass, and culminated on 10 April 2024 in a final report (source) that describes the evidence for medical treatment of minors with gender dysphoria as weak and recommends a different organisation of care. After the interim report of March 2022, NHS England had already decided to replace the Tavistock's GIDS service with regional services (source); GIDS closed at the end of March 2024. The NHS stopped routine prescribing of puberty blockers to under-18s in March 2024, and in December 2024 the UK government made the ban on private prescriptions indefinite (source).

More on this site: Cass Review (UK, 2024), University of York — Cass systematic reviews and UK National Archives — Cass appendices.

Background: the crisis surrounding GIDS and Tavistock

The Gender Identity Development Service (GIDS) at the Tavistock and Portman NHS Foundation Trust was founded in 1989 and for a long time was the only national service for gender care for children and young people in England. Before 2009 GIDS saw fewer than fifty children a year; from 2014 referrals rose exponentially, to roughly 5,000 in 2021/22 by its own count, a figure that according to Cass may include double counting. The majority were young people registered female at birth who presented in their early teens (source). People from within the service also came forward with concerns, including psychiatrist David Bell, who as a staff governor wrote an internal report in 2018 calling GIDS 'not fit for purpose', and board member Marcus Evans, who resigned in response (source).

In the Bell vs Tavistock case — brought by detransitioner Keira Bell — the British High Court ruled on 1 December 2020 that it is 'highly unlikely' that a child aged 13 or under can consent to puberty blockers, and doubted whether 14- and 15-year-olds can understand the long-term risks. In September 2021 the Court of Appeal overturned that ruling: it is for clinicians, not the court, to assess whether a child is competent (source). The Cass Review had already been commissioned in autumn 2020. In January 2021 the Care Quality Commission rated GIDS as 'inadequate' (same source). The clinic closed at the end of March 2024.

Methodology: systematic reviews through York

At Cass's request the University of York carried out eight systematic reviews (including two on guidelines) and an international survey; together these make up nine articles in Archives of Disease in Childhood (2024) (source). The full final report is available through the UK National Archives.

The reviews on puberty blockers (50 studies: 1 of high, 25 of moderate and 24 of low quality) and hormones (53 studies: 1 high, 33 moderate, 19 low) cover 103 studies in total; studies of low quality were not included in the synthesis (source). In her foreword Cass calls the field 'an area of remarkably weak evidence'. The review on puberty blockers concluded that no conclusions can be drawn about the effect on gender dysphoria, mental health and cognitive development (source). In its appraisal of guidelines, Cass writes that early versions of the Endocrine Society (2009) and WPATH 7 guidelines influenced nearly all other guidelines and that the two are closely interlinked; this 'circularity' may explain why there has been an apparent consensus although the evidence is poor (paras 9.20-9.22). According to Cass, only the Finnish (2020) and the Swedish (2022) guidelines could be recommended for use in practice (para 9.24; source).

Key findings

• The scientific evidence supporting medical treatment for gender dysphoria in minors is weak. For puberty blockers the York reviews found no change in gender dysphoria or body satisfaction, and insufficient or inconsistent evidence on psychological wellbeing, cognitive development, cardiometabolic risk and fertility; bone density falls during suppression. For hormones no conclusions could be drawn on gender dysphoria, psychosocial health, cognitive development or fertility, although there is suggestive evidence from mainly pre-post studies that hormone treatment may improve psychological health (paras 81-85; source).
• Puberty blockers are, according to Cass, not a proven pause button. In the British early intervention study 98 per cent (43 of 44) of young people on puberty blockers went on to hormones; Cass writes that there is no evidence that blockers buy time to think, and that there is concern they may change the trajectory of psychosexual and gender identity development (para 83; source).
• The patient group has changed: where previously mainly young children registered male at birth were referred, from around 2014 mostly teenagers registered female at birth came forward, with, according to the report, high levels of neurodiversity and/or co-occurring mental health issues (source).
• The interim report (2022) described 'diagnostic overshadowing': once gender-related issues have been identified in a young person, other important needs may be overlooked (source). According to the final report some clinicians dropped the usual broad assessment of the patient, and a diagnosis of gender dysphoria is not reliably predictive of whether someone will have long-standing gender incongruence or whether medical treatment is the best option (para 98; source).
• Information on long-term outcomes is lacking (paras 89-90). A data-linkage study of roughly 9,000 young people who went through GIDS did not get off the ground because six of the seven adult clinics were unwilling to cooperate; Cass called this very disappointing and recommended that NHS England direct the clinics to take part (recommendation 5; source). According to Cass it is unknown how many young people detransition after hormone treatment because long-term research is lacking (para 87); the review also spoke to people who had reversed their transition, and Cass recommends that NHS England ensure a service for people considering detransition (source).
• Social transition in children is described by Cass as an active intervention. There is no clear evidence that social transition in childhood has positive or negative mental health outcomes; Cass considers it possible that it changes the trajectory of identity development, and advises a more cautious approach for children than for adolescents, with clinical involvement of parents (paras 12.5, 12.31, 12.36; source).
• In the interim report, primary and secondary care providers said they felt pressure to adopt a purely affirmative approach, which clashed with the normal process of clinical assessment and diagnosis (source).
• According to the final report, young people with gender dysphoria have an elevated risk of suicide, but this appears comparable to that of other young people with similar mental health and psychosocial problems; the systematic review did not support the claim that gender-affirming treatment reduces suicide risk (para 15.36; source).
• For children who present early, gender incongruence according to Cass mostly resolves before puberty; in a small number it persists (source); see desistance.

Recommendations

• A network of regional centres with a broad, holistic assessment (recommendation 11), in which every case considered for medical treatment is discussed at a national multidisciplinary team (recommendation 9; source).
• Puberty blockers only under a research protocol (Cass's letter to NHS England, July 2023); the puberty blocker trial should be part of a full research programme (recommendation 6, same source).
• For hormones the option from age 16 is available, but Cass recommends 'extreme caution', with a clear clinical rationale for not waiting until 18 (recommendation 8, same source).
• A broad assessment of the needs of every referred young person, with screening for neurodevelopmental conditions such as autism and an assessment of mental health (source).
• All children should be offered fertility counselling and preservation before a medical pathway (recommendation 10, same source).
• A research programme on the characteristics, interventions and outcomes of every young person presenting to NHS gender services, with consent for follow-up into adulthood (recommendation 6), and participation of the clinics in the data-linkage study (recommendation 5).
• Services for 17- to 25-year-olds at every regional centre (recommendation 23; source).

Response from NHS England and the British government

NHS England published its response to the final report on 10 April 2024; the official NHS response describes the transition to regional centres and that puberty blockers are no longer routinely available within NHS gender care for children and young people. In December 2024 the UK government made the ban on the sale and supply of puberty blockers to under-18s via private prescriptions indefinite; the ban applies across the whole United Kingdom and will be reviewed in 2027 — see the British government announcement.

Implications for the Netherlands

The Cass Review is sensitive in the Netherlands because the Dutch model (the Dutch Protocol, developed according to Cass by Peggy Cohen-Kettenis) was followed internationally: in 2007 a clinic modelled on the Dutch approach opened in Boston (paras 2.13-2.14; source). The first study of puberty suppression, in 70 young people (de Vries et al., 2011), compared them before and during suppression and found no change in gender dysphoria or body satisfaction (source); the British study by Carmichael et al. (2021) likewise found no change in gender dysphoria or body satisfaction (para 82, same source as above). See also the Dutch Protocol.

In her letter to Parliament of 4 June 2024 the Dutch minister wrote that the conclusions of the Cass Review, where relevant to the Dutch context, will be taken into account in the revision of the quality standard; Radboudumc said it could support the recommendations, which have long been common practice in the Netherlands (source). On 30 June 2026 the Health Council of the Netherlands published its advice Transgender care for young people: it rates the certainty of the evidence on effects as low to very low, calls Dutch care carefully organised and sees no reason to organise it differently, and writes that the English literature on which the Cass Review relies is not directly applicable to the Dutch situation (source).

International impact

Cass calls the Finnish (2020) and Swedish (2022) guidelines the only ones that could be recommended for use in practice; both link the lack of robust evidence to a recommendation that treatments be provided within a research framework (paras 9.23-9.24; source). Sweden and Norway had already drawn their own conclusions before the final report: in December 2022 Socialstyrelsen gave a weak negative recommendation for puberty blockers, cross-sex hormones and mastectomy in minors (source), and in March 2023 Ukom recommended defining these treatments as 'utprøvende behandling' (experimental treatment) (source). In Australia, Queensland has since 28 January 2025 paused the initiation of puberty blockers and hormones for new patients under 18 in public healthcare.

Criticism and response

The Cass Review has also been criticised. On 17 May 2024 WPATH and USPATH accused the review of a 'selective and inconsistent use of evidence' (source). In 2024 a group around Meredithe McNamara (Yale School of Medicine) argued that the review's emphasis on 'high-quality' evidence is inappropriate, because in other parts of medicine too, much necessary care rests on evidence of moderate, low or very low quality (source); see the article on the Yale report and the legal cases in which it is being used. Noone et al. concluded in 2025 in BMC Medical Research Methodology that the systematic reviews for the Cass Review have a high risk of bias (source), while Cheung et al. wrote in Archives of Disease in Childhood that the review received broad support from the clinical community (source). On WPATH, The BMJ reported on 31 October 2024 that Johns Hopkins researchers who carried out evidence reviews for WPATH wrote that they were having issues with the sponsor trying to restrict their ability to publish; WPATH denied prohibiting publication (source); see the WPATH Files. For a more extensive discussion of the critical perspective, see critical perspectives on gender care.

More on WPATH, the Standards of Care and the WPATH Files: wpath.nl.