Should we have known? With hindsight there were plenty of clues. Katie had always been something of a tomboy – never a "girly" girl and most definitely not a Barbie girl – and we had begun to suspect that there was a possibility that, as adolescence progressed, she might turn out to be attracted to women rather than men. After all, my side of the family had previous form in this respect, so it crossed our minds and we did joke about it.

About two or three weeks before the bombshell dropped, an overheated dinner-table discussion (nothing unusual about that) erupted between Katie and Cass, my wife, about transgendered people and sexuality. It started with Katie talking about the film Boys Don't Cry, which she had seen recently. Katie seemed unusually animated by the issues and engaged in a discussion about the difference between gender identity and sexual orientation.

She exclaimed that she was astounded that her mother didn't know the difference; she did, but 15-year-olds in the heat of an argument are difficult to convince at the best of times. As the argument came to a head, Cass glared across the table and demanded of Katie whether there was something she needed to tell us. There was a slight pause, then a defiant "no" and we finished the meal in glum silence.

It must have been a few days later that Katie confided in Josh, her 14-year-old brother, that she no longer wished to be a girl and intended to start the transition towards becoming a boy. Two weeks later she plucked up the courage to tell her mother.

It must have been extremely difficult for Katie to speak to Cass about it, and it is to her credit that she did. When Cass passed the news on to me a few hours later, I was stunned but felt I had to go and say something to Katie. Perhaps I should have waited.

When I went into the study where she was doing a routine stint on Facebook, all I could think of to say was something along the lines of "Mum told me your news – it was really brave of you, and I'm proud of you." I gave her a hug, then added an excruciatingly embarrassing Dad joke along the lines of "but please ask before you borrow my razor".

Within seconds I felt I'd been clumsy and naff, but I'm not sure how better I could have done it. Then Cass and I hit the internet to do some research.

Gender dysphoria (also known as transgenderism, gender identity disorder or gender incongruence) is the medical term for the feeling that one's biology and gender identity are mismatched. Although still classified as a mental-health problem, increasingly the professional's view is that the root causes are likely to be organic, prompted by genetic predisposition or overexposure to particular hormones in the womb or both. For the person with the condition, it feels as if they are trapped in a body of the wrong sex.

One can only imagine the anxiety and discomfort this can produce, and the fear that the person is, somehow, not quite right. The NHS estimates that about one in 4,000 people seeks help with gender dysphoria but suggests there may be many more who do not.

On average, men are diagnosed with gender dysphoria five times more than women. Treatment may involve help to come to terms with the condition by means of dress and lifestyle, or result in hormone therapy and surgical intervention. It is often the case that pre-pubescent children who feel such dysphoria find it resolves itself before puberty. For those who enter adulthood with these feelings, the most common outcome is hormone treatment and surgery to alter their biological sex.

For adolescents under 18, the NHS process of treatment involves initial referral to Child and Adolescent Mental Health Services (CAMHS) and then, most likely, to a specialist gender-identity clinic. There are 10 NHS adult gender-identity clinics in the UK but only one for children and adolescents – the Gender Identity Development Service at the Tavistock and Portman NHS Trust in London. The relative rarity of the condition and information about support for it on the NHS all pointed to the likely probability of a protracted encounter with the medical profession.

We went to see our GP who, indeed, referred us to the local CAMHS, and they, in turn, after one consultation, referred Katie to the Gender Identity Development Service.

After the initial shock, my first feeling was one of grief. I was losing a daughter. She would, in time, become a son and no doubt be the same annoying but lovable teenager she is now. Nevertheless, the feeling that she was someone leaving us was palpable. For me, this feeling subsided in the days that passed, but for Cass it persists and is the cause of seemingly endless torment.

There is a complication for Cass, in relation to her religious beliefs, though I must stress that "complication" is my term and perspective, not hers.

I have reversed the situation in my mind: if Josh were to become Josie, would the impact on me be more severe? I find it impossible to judge even this hypothetical situation. My overriding urge is to protect and support, but without encouraging a headlong charge into anything quick or irreversible in the near future.

Case histories and the statistics on young people who transition mostly offer a range of accounts between the sobering and the alarming. There are some good news stories about successful transitions and happy lives, but equally there are many of anguish, pain, mental distress, self-harm and suicide. The key factor in determining good outcomes, it seems, is good levels of support from friends, family and health professionals.

The weeks after Katie's revelation were a rollercoaster of emotions. At the tail end of the Olympics, Katie and I went to London for the day to see an exhibition and go to Camden market. We had lunch and dinner in restaurants where, on both occasions, the waitress asked if "you gentlemen" would like dessert. To me, Katie is a girl trying to look like a young man, but to most other people she is just a young man. Afterwards, when I remarked on this and said it seemed a bit weird, she said, "Don't worry, Dad. It seems a bit strange to me too."

This situation has made me appreciate that we are fortunate to live where, and when, we do. Katie has just begun her A-levels at sixth form college and it was her wish to make a new start there with her new identity. Having filled in her application form as Katie, it was a mere administrative amendment to alter her name to Ben.

The college provided a sympathetic student tutor to chat things over with Katie, and that was that. At a pre-term student induction meeting Katie gravitated towards a group of boys and found immediate acceptance, though whether they knew I don't know and didn't ask. Interestingly, Katie's (Ben's) only comment was that boys seem to indulge in less inconsequential chat than girls, and that she found this refreshing. Bigger official hurdles inevitably lie ahead in the form of registration for public examinations, but one step at a time.

The family member who seems to be finding the situation most difficult to deal with is Josh. Katie confided in him first, but he didn't believe her. Over time, his position has moved to one of outright denial and a refusal to discuss the matter apart from the intermittent observation that "It's all fucked-up". Cass's and my best efforts to get him to open up and talk about how he feels have largely been rebuffed, though he has shown some softening of his stance in recent weeks.

Fear of what his schoolmates might say when they find out is clearly a significant issue, but we suspect this is only one worry Josh is prepared, or able, to admit to. As the second child, he's had a lifetime of living with someone who has already been there and done that two years earlier, and now at a critical time in his own transition from boy to man, via the choppy waters of adolescence, his elder sibling is now going to be "competing" in an area where he once had a clear, distinct and different identity.

The stress on the family has been intense; Katie recognises this and feels guilty, despite our reassurances that she isn't and shouldn't. This isn't just one person making a fundamental transition; this is an entire family in transition. Meanwhile, our three-year-old daughter remains an unaware and unconcerned island of calm.

The complications of work, travel and childcare arrangements meant that only Katie and myself could attend the first session at the Tavistock clinic. She has outwardly seemed to be coping very well and maintained this view until a quizzical eyebrow and some gentle, but insistent, probing by the therapist produced details about "bad days" where it was difficult to get out of bed and face the world with a body that seems alien. It's hard to imagine what this must feel like, but the discussion jolted me uncomfortably, and forced me to consider how utterly fundamentally Katie's sense of identity is now in opposition to her biology. Her account of dysphoria put me in mind of Kafka's Metamorphosis, where the protagonist, Gregor Samsa, wakes one morning to find that his body has been replaced by that of an insect. This is not just a teenager worrying about spots, or wishing she were taller, prettier or more blond.

Cass will be coming with us to the next session but Josh is still immovable and refuses to discuss the issue. This makes the impact on him harder to assess. He is a real worry.

Most parents might agree that not much can prepare you for raising a family, but for many of the situations, challenges, and outright puzzles that confront us there is advice to be found in parents and friends and a trusty copy of Penelope Leach's Your Baby & Child. Katie's situation is relatively unusual and deeply profound so it feels as if we are finding our own way with little to go on. Yet Cass and I both know families experience much worse, and we are still very much a family – just not quite the same one we were six months ago.

All names have been changed

Gender Identity Disorder

Gender Identity Disorder (GID) is the term given to people who feel they are in the wrong body. The first port of call is your GP, who will hopefully refer you to the local Child and Adolescent Mental Health Services team. You may then be referred to the Tavistock and Portman Clinic (you can also self-refer), the only place in the UK that deals with children with GID.

At the Tavistock and Portman, a full background and history is taken and extensive counselling is offered. There will be an endocrinology test to check for a hormone imbalance. "We don't take a view on whether GID will desist or persist. We try to ameliorate any negative effects and give them the space and opportunity to explore how they feel," says Dr Polly Carmichael, director of the Tavistock and Portman GID service.

Children with GID can become extremely distressed around the time of puberty when their bodies change. As well as extensive counselling, they may be offered a monthly injection of gonadotropin-releasing hormone (GnRH) commonly known as a blocker. This is entirely reversible, although the long-term effects are unknown.

In effect, the blocker puts puberty on pause and removes the psychological and hormonal distress of the changing body to allow the child to think about what he or she wants to happen next. Previously, this blocker wasn't available before the age of 16, but now, in line with Dutch protocol, it's offered earlier.

At about 16, this may be continued or stopped; or the young adult can then go on to cross-sex hormones.

At 18, the patient would be transferred to adult services, with the option of surgery if required. The Family Law Act says that a young person is a minor in terms of medical treatment until the age of 16. However, in GID cases treatment can be given earlier, without parental consent if the minor is deemed by a doctor to be competent and that they understand the decision they are making and the consequences of it. However, in practice it's extremely rare that a surgeon would perform surgery before the person is 18.

"It's hard to grasp the distress associated with GID," says Dr Carmichael. "And it's important to understand that the blocker is not the inevitable step to the next stage. By referring to us, someone is not on the inevitable road to change."

Annalisa Barbieri

• Support and information: pfc.org.uk, tavistockandportman.nhs.uk/ childrenyoungpeoplegenderidentity issues, mermaidsuk.org.uk