Stress doesn't cause hairloss. Even med school stress. Genetics causes hairloss. If there weren't a bunch of bald men coming home from Germany, Vietnam, Korea, Afganistan, and Iraq after fighting wars, med school won't cause it.
If I were you, i would get on finasteride and you will still be stable when you graduate. If you become a surgeon or anesthesiologist, you will wear a head cover anyway and can get an HT without worry!