You may be pleasantly surprised with the reaction of your 16 year old. In this day and age, most kids that age have at least one friend who is openly gay or lesbian, or maybe more than one, and some even have a friend, or a friend of a friend, who is transgender.
Remember, roughly 1 out of every 300 people is transgender. I am told the average person has around 300 Facebook friends.
So I would say the odds are pretty good that your daughter has some kind of real world experience with gender variance, and kids that age are pretty tolerant for the most part.
I would not worry about it.