A few days ago, I was doing some internet research spurned by noticing that a lot of my trans girl friends seemed to have very androgynous eunuchoid body shapes. I went on a research binge to see if it's possible that disorders which result in hypogonadism, like mild AIS and Klinefelter's and the like, resulted in higher rates of transsexualism. And then I started looking to see if hyperandrogenic disorders like CAH and PCOS were more common among trans guys.
While I'm still trying to find medical studies on the former, I did find a study on the later, and I thought it was really enlightening, so I thought I'd share it.
http://www.ncbi.nlm.nih.gov/pubmed/9279941This was admittedly a small sample size, but in terms of results... wow.
Transgender men in this study, before any medical intervention at all, naturally had 31% higher total testosterone levels than cisgender women on average. (54.0 vs. 41.1,) and a whopping 173% higher levels of free testosterone (testosterone unbound by Sex-Hormone-Binding Globulin,) with trans men having average levels of 72.0 and cis women having average levels of 26.4.
More and more studies showing that there's a lot more than just social factors going on in people with trans identities, there's a component which is very much biological.