I don't know where it originates, but I don't think it's a medical claim so much as a an "ameliorative definition": a redefinition for political purposes. I don't know if these philosophers created the ideas or just reflect ideas that have emerged through activism, but you can see some turning points perhaps in two philosophical papers. In Sally Haslangers "Gender and Race" -- in which she argues that 'women' should be redefined in terms of social subordination based on perceived femaleness rather than on femaleness itself. Her idea is that this will help the word 'woman' do the work of picking out those individuals who are the concern of feminism. The second paper is Katharine Jenkins's "Amelioration and Inclusion" in which she argues woman should actually be defined as a gender identity so that no one is categorised as man or woman against their preference, and so that we can centre trans women in feminist spaces.

More recently I feel a lot of the discourse has evolved online, so it may be difficult to pin down the origins of different ideas. But you should also look at the advocacy work of Stonewall and Mermaids, for example, since they receive money to train people on trans issues.