Ever thought about a gender reassignment where you substitued male for female characteristics or the opposite?
I haven't for myself, but I know people who have been in that situation.  I'm all for it.
Did your friend do hormone therapy?
He did, yeah.  Still going through it.  He was born a biological female, and for the longest time thought he was just a lesbian.  But something wasn't quite right still.  Admirable person.
Has there been surgery to modify some characteristics?
Not yet.  Not sure what he's considering, and it's kind of private, so I haven't asked.  What do you know about the subject?
I know that it can be a result of intersex at birth.
Is that so?  Have their been scientific studies on it?
Intersex
Intersex people are individuals born with any of several variations in sex characteristics including chromosomes, gonads, sex hormones, or genitals that, according to the UN Office of the High Commissioner for Human Rights, "do not fit the typical definitions for male or female bodies". Such variations may involve genital ambiguity and combinations of chromosomal genotype and sexual phenotype other than XY-male and XX-female.
Intersex people were previously referred to as hermaphrodites or "congenital eunuchs". In the 19th and 20th century,
people born with physical sex characteristics "that do not fit typical binary notions of male or female bodies". The opposite of "transgender" is "cisgender", which describes persons whose gender identity or expression matches their assigned sex.
The degree to which individuals feel genuine, authentic, and comfortable within their external appearance and accept their genuine identity has been called "transgender congruence". Many transgender people experience gender dysphoria, and some seek medical treatments such as hormone replacement therapy, sex reassignment surgery, or psychotherapy.