The cause of gender dysphoria is psychological or psychiatric in origin and not physiological. One isn't "assigned" a sex at birth. Apart from exceptionally rare medical conditions, we are born male or female regardless of what we might "feel" about this as we grow older. 

One's sex - male or female - is a natural fact of life, not a social construction or matter of personal choice. 
There is a Catholic teaching on this. Briefly stated:

•each of us is made in God’s image as man or woman;
•gender identity is determined at conception, genetically, anatomically and chromosomally;
•a person must accept that objective identity.

Catholic teaching is clear on the need to accept the objective truth of the reality of our bodies in this condition, and therapy should have that aim as its goal.