Or at least becomes less necessary?
After Gender Reassignment surgery and the glands which produce the hormones are removed wouldn't your t or e levels stop entirely?
I have no medical knowledge, sorry if I am speaking out of any other orifice aside from my mouth
You must have normal levels (for your age) of some sex hormone in order to maintain bone strength. It doesn't seem to matter terribly much which one it is, so people who retain their original gonads can start and stop HRT without too much risk of ill effects, but if you have an orchi or oophorectomy then HRT is medically necessary for the rest of your life.