There's a tendency among TG & Ts's to want to be intersexed, or have some medical diagnosis that explains why they are the way they are. I've knew one TS who was absolutely certain she was intersexed or Kleinfelters (XXY chromosomes, a type of intersex condition). But alas, she was "normal" (what ever that means!).
It would be much easier to gain acceptance for transgender/transsexualism if they could be shown to be physiological. Since there's no smoking gun, we get the "It's just a lifestyle, you can cure it with willpower, or faith" argument from the anti crowd.
I have whistfully contemplated the possibility that I'm the way I am because of DES -- after all, I have a super femme digit ratio (index fingers 4mm longer than my ring fingers). Somehow that's a comforting thought -- it would prove that I'm not neurotic or obsessive.
Note that 3-5% of full term boys have undescended testicle, so it's not exactly a rare or drug caused condition.
I do believe that those of us who are transgendered and TS have a neurological intersex condition. But it's probably very subtle and may not have a genetic component.
The Transgendered have been with us for a long time, before hormones got into drinking water. I think it's a naturally occuring variation, like gayness or green eyes or left-handedness (the latter two I'm afflicted with). You know, the whole tapestry of human life thing