Taking testosterone itself can override the effects of estrogen, which is why taking e-blockers is not a common part of long-term hormone replacement therapy. T is just far more potent a hormone (which is why MTFs can't "undo" many effects of puberty by taking estrogen). And as noted above, if an FTM has a hysterectomy, it will end estrogen production, so that person can take a lower dose of testosterone, because it is no longer "competing" with the estrogen to have an effect. Until then, you don't have to block estrogen, because Testosterone acts as an e-blocker of sorts.
It sounds like some endocrinologists now think there are advantages to blocking estrogen first before introducing T. Outside of the cases of younger teens, so far as I can tell the three reasons for e-blocking before T are:
(1) To get rid of periods/monthly bleeding earlier
(2) To allow for a greater androgenic response to a lower start-up dose of T;and
(3) and to eliminate the likelihood of the need for follow-up medical care to deal with "questionable" events that may be a normal/natural reaction to the introduction of T or could indicate a more serious complication.
The biggest problem with an extended period of e-blocking prior to T is that it compromises bone density and increases risks of osteoperosis.
