Hi,
I am both trans and IS, and i do consider typical cases of transgender identity and transition (I.e., identifying as the gender opposite one's assigned at birth gender,hormone replacement therapy, and social transition) to usually be a form of intersex. I had to qualify that statement twice though with words like "typically" and "usually". Sex, like gender, is a spectrum, and I think that although trans people are often not visibly intersex prior to taking hormones, that there is evidence that the nuerological differences typical in trans people are a form of sexual variation (intersex). Also, there are cases where trans people have other, undiagnosed, invisible intersex characteristics. Taking hormones absolutely makes one visibly, and undeniably intersex.
Where this gets tricky though is that there are a plethora of complicated emotional, political, cultural, and sexual factors that influence variant gender expression, transgender identity, and the decision to transition. It should also be noted that homosexuality is also frequently associated with sexual variations in neurological make up, as well as variant gender expression without transgender identity, gender transition or visible intersex characteristics. Can this also be considered a form of intersex? I think, by the lose definition of intersex that one uses to include trans people, it can (its interesting to think about where butch lesbian trans women and femme gay trans men fit on this spectrum, to say the least).
Also, despite some overlap in issues that are common to persons who were either born visibly intersex or spontaneously developed intersex characteristics during puberty and transgender identity (such as hormones, surgery, passing, legal issues, and the fact that gender transition is relatively more common amongst the intersexed), it is useful to have the concepts of trans and intersex separate in order to discuss what is different about them (involuntary intersex surgeries, involuntary hormone therapy, the unique issues that affect cis intersex persons like passing as one's assigned at birth gender, etc).
I'm not offended by trans people who were not born visibly intersex and only became visibly intersex via hormones and/or surgery (or not at all) identifying as intersex, but it is still important to me to be able to make a distinction between typical intersex and typical trans experiences. Personally, my intersex identity is more important to me than my trans identity in describing my experiences with sex and gender (and I simply cannot relate to the standard trans narrative as I have socially transitioned without hormones and" pass" better than I did prior to transition), although the unique language of the transgender experience also describes my own experiences in ways a purely intersex narrative cannot.