Bookworm looks at some of the research on the effects of sex hormones.
I think there are true transgender individuals, but they represent the extreme tail end of the normal distribution curves. But I’ve been in conversations with people who cite these extreme examples and “reason” therefrom that gender is a meaningless social construct. So it’s an error to claim that there are two sexes. It’s probably also an error to claim that the sexes differ, and probably an error to believe that fiddling with a person’s levels of sex hormones would have any negative impact. (How this can be reconciled with the necessity of such fiddling in the first place is yet to be explained.)