breast not developed

my niece is 14 and one breast is more than one cup smaller, is it due to an endocrinological issue?

[Deanna L Aftab Guy, MD]

not necessarily, I have seen a few kids with this, workup all normal hormone levels and mostly due to less actual tissue itself in that area, tend to monitor them with their pediatricians and once they are all done developing by age 18 at least if there is a big discrepancy they can make a decision if reduction of the larger size is needed, of course ruling out any mass on the larger size is important but physical exam should help.
So start with the pediatrician or adolescent specialist