Your Teenage Child tells you they think they are Transgender

It won't happen.
Mine are growing up able to recognise that stereotypes are meaningless, every person has a unique personality, and that there are male and female bodies. End of.
They won't suddenly start thinking that all they are the opposite sex inside their heads, because they know that isn't possible.
There are girl and boy bodies, but not girl and boy minds.
We talk about gender the way we talk about religion. That some people believe in many gods, some in one, and some don't believe at all. That you should be kind to people with different beliefs but you never have to pretend you believe the same thing, and that beliefs can only take you so far, before they butt up against reality. Which always wins. Also that sometimes people's beliefs can be nasty about you and very untrue, and you are definitely allowed to disagree with their opinion of you and your sex in general.

Assume ROGD and help them to be their true self (without irreversible medical procedures).

Change the WiFi password, buy a cheap non-smart phone for them. Remove internet access. Let them express themselves how they want, no drama. Don't make it a big thing/main topic of conversation. Reassure that they're loved. Watch and wait. If feelings persist for longer (with no internet), or are causing distress, find a decent counsellor.

I would thank them for sharing/ opening up, and reiterate that my role is to help them navigate life's complexities.

I would do some normalising and that it is common to think about sex, gender and identity during the teenage years. I would explain that adolescence is all about exploring different identities. I would reassure them that it is OK to feel this way and that they should continue to explore and think about who they are and what they want to be.

I’d want to find out what had made them feel like they were unhappy in their own body. I’d be reinforcing that it’s fine for boys to wear dresses and have long hair. And asking them whether they were same-sex attracted and obviously reiterating that I wouldn’t have a problem with it if they were. And just generally keeping lines of communication open and cutting down on their internet usage!

Conversation. Probably best kept low key ( in the car for example) direction would be History dependent i.e. Does that fit with my understanding and observations of the child or this is out of the blue? What else is going on in their life? How old are they? How much does it matter? If they are early teens and talking about drugs off the internet for example it may become a more challenging situation where I might seek advice - probably a doctor in the first case. I may just listen or gently prod or recount my own experiences of growing up. If it seemed serious I would probably feel sad and worried as you always want they easy path in life for your child.

Low key 'OK' and then wait and see what actually happens.

Given one DC's quoting of the 'attack helicopter" meme, I'd expect pretty serious intent before saying anything to family. And of course you can't really stop teens doing exactly what they want to. But you can be there, come what may.

What would yours be?

Yes dear very funny

Seriously though I would talk it through with her and discuss why she feels that way