By James Fletcher
Around 1 in 4,000 people in the UK are receiving help for gender dysphoria
Doctors in Britain are reviewing guidelines for the treatment of people under 18 with gender dysphoria. This is a condition where someone is born one sex but feels they are really the other. A key issue is the age at which young people can be prescribed drugs which pause puberty.
Sitting in her kitchen, 16 year old Nikki (name changed to protect her identity) looks and acts like any other teenage girl.
She gossips with her mum, teases her younger brothers, and giggles as she texts her new boyfriend.
The only difference is that Nikki was born biologically a boy.
"I've always felt like a girl," she says. As a child, she dressed up in girls' clothing, played with girls' toys, and gravitated towards other girls.
As she got older, Nikki realised there was a difference between what she felt and her body.
"It used to make me feel ill and so horribly down," she remembers. "I'd just wish that I wasn't around. Everything to do with me being male was horrible, I just couldn't stand it."
When she was seven Nikki was diagnosed with gender dysphoria. Her parents initially tried to steer her towards accepting she was a boy, but by eight she was living as a girl at home, and by nine was going to school as a girl.
"I loved it," she says, "people picked on me a lot, but it was amazing in my eyes because I was allowed to show everyone who I was."
Terrified and distressed
Gender dysphoria is where a person feels they are trapped within a body of the wrong sex. It is also known as gender identity disorder, or transgenderism.
People with long lasting, extreme cases of gender dysphoria are known as transsexuals.
Around one in 4,000 people in Britain is receiving medical help for gender dysphoria.
The Tavistock clinic says it gets around 100 new referrals of people under 18 a year.
Countries where hypothalamic blockers are offered in the early stages of puberty include the Netherlands, Germany, Belgium, United States, Canada and Australia.
International guidelines from the Endocrine Society and the World Professional Association for Transgender Health recommend the use of blockers in the early stages of puberty
Sources: NHS, Tavistock clinic
As Nikki approached her teens, things became more difficult. The bullying got a lot worse, and puberty was about to begin.
"That was scary," recalls Nikki. "I do love to sing, so the thought of my voice breaking was horrible."
Most young people who experience gender issues before puberty end up remaining in their biological sex.
Experiencing their own sex hormones at puberty can be a key factor in this.
Nikki's mum Sharon (name changed to protect identity) hoped that this would be the case for Nikki, but the possibility of her body changing had the opposite effect.
"She was just so terrified and very very distressed," says Sharon.
"Her conviction that she would rather die became very much more clear during that point, it was very strong - I'm not going to live through this I'd rather be dead."
One treatment for young people with strong gender dysphoria is drugs known as hypothalamic blockers. These block an individual's sex hormones, suspending puberty. If the drugs are stopped, puberty resumes.
Over the past ten years, doctors in several countries have begun prescribing blockers in the early stages of puberty, at around 12 years old. This gives adolescents more time to explore and make decisions about their identity.
This approach was pioneered by the Amsterdam Gender Clinic for Children and Adolescents in the Netherlands.
"If you wait too long, the adolescents go through their puberty and have changes they don't want to have," says the clinic's head, Professor Peggy Cohen-Kettenis.
In Britain blockers are prescribed by the government-funded Tavistock Gender Identity Development Service. Their current policy is to use them later in puberty, at around 16.
"There are very good reasons put forward for early intervention," says the Tavistock's director, Dr Polly Carmichael. "However it is important to bear in mind that this is still a relatively new intervention."
Dr Carmichael says concerns include the effects of early intervention on physical development, and that it can preclude fertility.
"There is also an issue around the impact of sex hormones on brain development," she adds. "If one stops pubertal hormones very early on in puberty, is one in some way altering the course of nature?"
Research from the Dutch clinic has shown positive outcomes so far, and Dr Carmichael says the Tavistock plans to start offering blockers earlier in puberty as part of a research project.
According to Professor Cohen-Kettenis, doctors have to strike a difficult balance.
"In general interfering with development is difficult if you don't completely understand why," she says, "and doctors' main concern is 'first do no harm.' But we expect that it will be even more harming if you do not do anything."
Sharon and Nikki felt that waiting until 16 was not an option and decided to go to the United States where Nikki was prescribed blockers at 13.
They want Britain to follow international guidelines which recommend the use of blockers earlier in puberty.
'My female life'
When Nikki talks about her life now, her mood visibly changes. She smiles more and bounces in her seat with excitement.
She has recently started studying at a new college where no one knows she was born a boy.
"College was the start of my female life, my girl life properly. I'm happy."
In line with British and most international guidelines, Nikki began taking female hormones at 16 and plans to have gender reassignment surgery when she is 18.
Although the blockers are reversible, they can be seen as the beginning of the road towards a sex change, and there are concerns about whether people aged 12 or 13 can make informed decisions about embarking on this process.
But Nikki says she knew what she was doing: "It's your own body, it's your choice, it's not anyone else's to decide for you, because that's just taking away all the control that people want to have and people need to be sane."