The topic of teens wanting to change genders is a very delicate one and also one without much information available. There are plenty of people who want to argue their ideological viewpoint, but that isn't helpful when you're dealing with a real situation with someone you love.
The key term being discussed is Gender Dysphoria.
Gender Dysphoria involves a conflict between a person's physical or
assigned gender and the gender with which he/she/they identify
An Article in Quillette, which I think your daughter should read, states:
In the past decade, however, a new presentation of gender dysphoria
has suddenly become widespread, in which teens or tweens come to
identify as transgender “out of the blue,” without any childhood
history of feeling uncomfortable with their sex. Experts have dubbed
this presentation rapid onset gender dysphoria, and are beginning to
The American Psychiatric Association states
For many children the feelings do not continue into adolescence and
People with gender dysphoria have higher rates of mental health
The previous 3 statements are not made for any other reason than to point out the seriousness, complexity, and the gentleness that this issue should be taken with. They also both serve as further resources a reader may use to do their own research.
With this minimal amount of information about the topic I'd like to offer some reasonable information on how to handle it. I'm going to the anecdotal research by a Social Worker with experience in these issues. The source information is not a long read and I suggest you simply go read it yourself. There are 2 posts that discuss having a teen with Gender Dysphoria (the author is the same author who wrote the article in Quillette above):
Don't fall into the trap arguing about gender dysphoria. Keep in mind how much you love your child as you go through this. Try to ask questions and understand why your child feels this way.
The ideology of innate gender identity doesn’t make much sense of a
lot of the time, and many young people seem to sense this
At all costs avoid shaming or punishing your child over gender identity. At the same time, you can choose to not agree to the identity. Choosing your response needs to be a unique choice based on personal information no one here will have. Part 1 discusses this in more detail.
Challenging a child’s beliefs should always be done without anger. As
much as possible, it ought to be done respectfully and with genuine
empathy for the child’s suffering. Some parents have had good success
with carefully timed, short reality-based statements, delivered with
authority but not anger
Remember children are children because they simply are not ready to live on their own. They don't know enough and they are not mature enough. It is your responsibility to be in charge and keep your child safe during this time.
It is really hard to research this stuff on your own when your world feels like it is falling apart. I genuinely believe the content posted by Lisa Marchiano in the 3 main links is reasonable. It is also all I have been able to find. This is a very new phenomenon.
Note on Social Media
There is strong evidence about the negative influence of social media on teens and tweens. That is not the main topic at hand, but from your question it seems to be a major impact on this specific situation. There are other questions which have answers to this topic.
More Personal to you
I could give you many more opinions, but there are many questions. I wonder how much you know about the therapist you've chosen, how much you know about what your daughter does on social media, and of course what core issues were that led to the therapist in the first place. There is a lot for you to unpack, my questions are rhetorical for you to answer.
Appendix 1: American Academy of Pediatrics
Further reading is a PDF printed by the Children's Hospital of Chicago by the American Academy of Pediatrics. They officially support Affirming gender change in children, however there is a group of pediatricians who actively reject that position.
At the least, if your daughter does start to consider therapy in the future with or without your consent, please review pages 39-41 and see the side effects of testosterone therapy.
Irreversible Effect of Testosterone Therapy:
Increased Hair Growth
Mustache and Beard Growth
Male Pattern hair loss and baldness
Loss of Fertility
Side Effects of Testosterone Therapy;
Increased Weight gain
Increased Blood Pressure
Hepatotoxicity (liver disease)
Polycythemia (increase change of heart attacks, blood clots, and more)
Increased risk for breast cancer
Increased risk of Endometrial cancer
Post Research Note
It appears there is a good amount of support for Gender Dysphoria in some national groups like the APA. However, no reading of the definition of Gender Dysphoria has shown to be anything other than supporting what people feel. I cannot find genuine objective information on the topic.
Post Research Note 2
Parenting Stack Exchange is not the place for questioning the topic of Gender Dysphoria in general. I opened a self-admittedly long winded question on the Psychology & Neuroscience Stack Exchange site where I address some of my concerns over the general validity of the APA holding a stance that supports and affirms gender reassignment, especially in children.
Post Research Note 3
I think there is a huge gap between the reality of this topic and how we talk about and understand it. I have posted a position on this gap at the Psychology & Neuroscience Stack Exchange site here.