Yes, it has clearly been proven to be. 

Wired, not chosen

Studies using high resolution functional MRI in hormone-naïve patients (meaning patients who have not taken feminizing hormones) clearly reveal characteristic differences from those of non-transsexuals.  These neuroimging scans are far more sophisticated than regular MRIs, like looking at a 4K television set, and they are dynamically functional instead of statically structural.  They confirm my early observation that transsexuality must be hard-wired, since I had noticed that about 25% of my patients were left-handed and/or ambidextrous, a very high proportion. I will publish the letter I received back in the 80s or 90s acknowledging this observation, at a later date on this web-site.