After 30 years as one of England’s top pediatricians, Dr. Hilary Cass was hoping to begin her retirement by learning to play the saxophone.

Instead, she took on a project that would throw her into an international fire: reviewing England’s treatment guidelines for the rapidly rising number of children with gender distress, known as dysphoria.

At the time, in 2020, England’s sole youth gender clinic was in disarray. The waiting list had swelled, leaving many young patients waiting years for an appointment. Staff members who said they felt pressure to approve children for puberty-blocking drugs had filed whistle-blower complaints that had spilled into public view. And a former patient had sued the clinic, claiming that she had transitioned as a teenager “after a series of superficial conversations with social workers.”

The National Health Service asked Dr. Cass, who had never treated children with gender dysphoria but had served as the president of the Royal College of Pediatrics and Child Health, to independently evaluate how the agency should proceed.

Over the next four years, Dr. Cass commissioned systematic reviews of scientific studies on youth gender treatments and international guidelines of care. She also met with young patients and their families, transgender adults, people who had detransitioned, advocacy groups and clinicians.

Her final report, published last month, concluded that the evidence supporting the use of puberty-blocking drugs and other hormonal medications in adolescents was “remarkably weak.” On her recommendation, the N.H.S. will no longer prescribe puberty blockers outside of clinical trials. Dr. Cass also recommended that testosterone and estrogen, which allow young people to develop the physical characteristics of the opposite sex, be prescribed with “extreme caution.”

Dr. Cass’s findings are in line with several European countries that have limited the treatments after scientific reviews. But in America, where nearly two dozen states have banned the care outright, medical groups have endorsed the treatments as evidence-based and necessary.

The American Academy of Pediatrics declined to comment on Dr. Cass’s specific findings, and condemned the state bans. “Politicians have inserted themselves into the exam room, which is dangerous for both physicians and for families,” Dr. Ben Hoffman, the organization’s president, said.

The Endocrine Society told The New York Times that Dr. Cass’s review “does not contain any new research” that would contradict its guidelines. The federal health department did not respond to requests for comment.

Dr. Cass spoke to The Times about her report and the response from the United States. This conversation has been edited and condensed for clarity.

What are your top takeaways from the report?

The most important concern for me is just how poor the evidence base is in this area. Some people have questioned, “Did we set a higher bar for this group of young people?” We absolutely didn’t. The real problem is that the evidence is very weak compared to many other areas of pediatric practice.

The second big takeaway for me is that we have to stop just seeing these young people through the lens of their gender and see them as whole people, and address the much broader range of challenges that they have, sometimes with their mental health, sometimes with undiagnosed neurodiversity. It’s really about helping them to thrive, not just saying “How do we address the gender?” in isolation.

You found that the quality of evidence in this space is “remarkably weak.” Can you explain what that means?

The assessment of studies looks at things like, do they follow up for long enough? Do they lose a lot of patients during the follow-up period? Do they have good comparison groups? All of those assessments are really objective. The reason the studies are weak is because they failed on one or more of those areas.

The most common criticism directed at your review is that it was in some way rigged because of the lack of randomized controlled trials, which compare two treatments or a treatment and a placebo, in this field. That, from the get-go, you knew you would find that there was low-quality evidence.

People were worried that we threw out anything that wasn’t a randomized controlled trial, which is the gold standard for study design. We didn’t, actually.

There weren’t any randomized controlled trials, but we still included about 58 percent of the studies that were identified, the ones that were high quality or moderate quality. The kinds of studies that aren’t R.C.T.s can give us some really good information, but they have to be well-conducted. The weakness was many were very poorly conducted.

There’s something I would like to say about the perception that this was rigged, as you say. We were really clear that this review was not about defining what trans means, negating anybody’s experiences or rolling back health care.

There are young people who absolutely benefit from a medical pathway, and we need to make sure that those young people have access — under a research protocol, because we need to improve the research — but not assume that that’s the right pathway for everyone.

Another criticism is that this field is being held to a higher standard than others, or being exceptionalized in some way. There are other areas of medicine, particularly in pediatrics, where doctors practice without high-quality evidence.

The University of York, which is kind of the home of systematic reviews, one of the key organizations that does them in this country, found that evidence in this field was strikingly lower than other areas — even in pediatrics.

I can’t think of any other situation where we give life-altering treatments and don’t have enough understanding about what’s happening to those young people in adulthood. I’ve spoken to young adults who are clearly thriving — a medical pathway has been the right thing for them. I’ve also spoken to young adults where it was the wrong decision, where they have regret, where they’ve detransitioned. The critical issue is trying to work out how we can best predict who’s going to thrive and who’s not going to do well.

In your report, you are also concerned about the rapid increase in numbers of teens who have sought out gender care over the last 10 years, most of whom were female at birth. I often hear two different explanations. On the one hand, there’s a positive story about social acceptance: that there have always been this many trans people, and kids today just feel freer to express who they are. The other story is a more fearful one: that this is a ‘contagion’ driven in large part by social media. How do you think about it?

There’s always two views because it’s never a simple answer. And probably elements of both of those things apply.

It doesn’t really make sense to have such a dramatic increase in numbers that has been exponential. This has happened in a really narrow time frame across the world. Social acceptance just doesn’t happen that way, so dramatically. So that doesn’t make sense as the full answer.

But equally, those who say this is just social contagion are also not taking account of how complex and nuanced this is.

Young people growing up now have a much more flexible view about gender — they’re not locked into gender stereotypes in the way my generation was. And that flexibility and fluidity are potentially beneficial because they break down barriers, combat misogyny, and so on. It only becomes a challenge if we’re medicalizing it, giving an irreversible treatment, for what might be just a normal range of gender expression.

What has the response to your report been like in Britain?

Both of our main parties have been supportive of the report, which has been great.

We have had a longstanding relationship with support and advocacy groups in the U.K. That’s not to say that they necessarily agree with all that we say. There’s much that they are less happy about. But we have had an open dialogue with them and have tried to address their questions throughout.

I think there is an appreciation that we are not about closing down health care for children. But there is fearfulness — about health care being shut down, and also about the report being weaponized to suggest that trans people don’t exist. And that’s really disappointing to me that that happens, because that’s absolutely not what we’re saying.

I’ve reached out to major medical groups in the United States about your findings. The American Academy of Pediatrics declined to comment on your report, citing its own research review that is underway. It said that its guidance, which it reaffirmed last year, was “grounded in evidence and science.”

The Endocrine Society said “we stand firm in our support of gender-affirming care,” which is “needed and often lifesaving.”

I think for a lot of people, this is kind of dizzying. We have medical groups in the United States and Britain looking at the same facts, the same scientific literature, and coming to very different conclusions. What do you make of those responses?

When I was president of the Royal College of Pediatrics and Child Health, we did some great work with the A.A.P. They are an organization that I have enormous respect for. But I respectfully disagree with them on holding on to a position that is now demonstrated to be out of date by multiple systematic reviews.

It wouldn’t be too much of a problem if people were saying “This is clinical consensus and we’re not sure.” But what some organizations are doing is doubling down on saying the evidence is good. And I think that’s where you’re misleading the public. You need to be honest about the strength of the evidence and say what you’re going to do to improve it.

I suspect that the A.A.P., which is an organization that does massive good for children worldwide, and I see as a fairly left-leaning organization, is fearful of making any moves that might jeopardize trans health care right now. And I wonder whether, if they weren’t feeling under such political duress, they would be able to be more nuanced, to say that multiple truths exist in this space — that there are children who are going to need medical treatment, and that there are other children who are going to resolve their distress in different ways.

Have you heard from the A.A.P. since your report was published?

They haven’t contacted us directly — no.

Have you heard from any other U.S. health bodies, like the Department of Health and Human Services, for example?

No.

Have you heard from any U.S. lawmakers?

No. Not at all.

Pediatricians in the United States are in an incredibly tough position because of the political situation here. It affects what doctors feel comfortable saying publicly. Your report is now part of that evidence that they may fear will be weaponized. What would you say to American pediatricians about how to move forward?

Do what you’ve been trained to do. So that means that you approach any one of these young people as you would any other adolescent, taking a proper history, doing a proper assessment and maintaining a curiosity about what’s driving their distress. It may be about diagnosing autism, it may be about treating depression, it might be about treating an eating disorder.

What really worries me is that people just think: This is somebody who is trans, and the medical pathway is the right thing for them. They get put on a medical pathway, and then the problems that they think were going to be solved just don’t go away. And it’s because there’s this overshadowing of all the other problems.

So, yes, you can put someone on a medical pathway, but if at the end of it they can’t get out of their bedroom, they don’t have relationships, they’re not in school or ultimately in work, you haven’t done the right thing by them. So it really is about treating them as a whole person, taking a holistic approach, managing all of those things and not assuming they’ve all come about as a result of the gender distress.

I think some people get frustrated about the conclusion being, well, what these kids need is more holistic care and mental health support, when that system doesn’t exist. What do you say to that?

We’re failing these kids and we’re failing other kids in terms of the amount of mental health support we have available. That is a huge problem — not just for gender-questioning young people. And I think that’s partly a reflection of the fact that the system’s been caught out by a growth of demand that is completely outstripping the ability to provide it.

We don’t have a nationalized health care system here in the United States. We have a sprawling and fragmented system. Some people have reached the conclusion that, because of the realities of the American health care system, the only way forward is through political bans. What do you make of that argument?

Medicine should never be politically driven. It should be driven by evidence and ethics and shared decision-making with patients and listening to patients’ voices. Once it becomes politicized, then that’s seriously concerning, as you know well from the abortion situation in the United States.

So, what can I say, except that I’m glad that the U.K. system doesn’t work in the same way.

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When asked after this interview about Dr. Cass’s comments, Dr. Hoffman, the A.A.P.’s president, said that the group had carefully reviewed her report and “added it to the evidence base undergoing a systematic review.” He also said that “Any suggestion the American Academy of Pediatrics is misleading families is false.”