Make it clear to your daughter that it's entirely her choice how she wants to live her life when she is an adult. Tell her you will stand by her regardless. 

Maybe she wants to be called "he" from now on, I wouldn't deny this. If you do she/he will probably distance himself/herself from you. Maybe he/she wants to have some boy clothes, buy them.

The drugs in question have adverse effects on bone mineralisation and compromise fertility; data on the effects on brain development are still limited [\[1\]][1]. The desired effects of the drug are reversible though [\[1\]][1].

I think it's important to realize that she can't make the choice to get therapy, no matter how bad she/her wants it. And this for good reason. She is still very young and her views on many topics not yet fully established.

Advise her/him to first try her new lifestyle at home in a safe space for some time. You should make it very clear to her what the possible consequences may be in the outside world, while stressing that you stand with her/him regardless. 

Prevent her/him from rushing things. This process will take years and children change. You want to prevent her from doing something she later regrets and stretch the process out over many years in order to do so. Be clear about that: Tell her this is not going to happen in a matter of days, weeks or months. 

The most important thing is that you need to have long talks about identity. Soley changing gender will probably not make her happy. I for myself often thought that getting a top grade in university, a certain well-paid job or a new partner will make me happy, but this has been naive. Happiness is nothing you acquire by some surgery or some single event.


  [1]: https://www.sciencedirect.com/science/article/pii/S2444866417301101