Have a study.
https://www.sciencedaily.com/releases/2015/02/150224143110.htmIf you search for it I'm sure there's more, but this one is recent and apparently "the most comprehensive to date".
Spoilers: Medical professionals who know what they're talking about consider correctly administered HRT to be safe. There's some risk of side effects, but that can be said of many medications that people take to improve their quality of life.
Here's an interesting article:
http://thinkprogress.org/lgbt/2015/06/22/3672506/transgender-suicide-rates/Appropriately administered HRT is relatively safe. Lack of treatment, discrimination, etc. not so much.
Even setting all that aside, there's the matter of personal choice and quality of life.
For me top surgery + T definitely extended rather than decreased my lifespan. The genital dysphoria still gets me in a major way sometimes, but I am overall happier and more functional than I ever was before. I am a happy and productive man instead of a miserable shut-in. If I had known beforehand it would be this big a difference in quality of life, but it would kill me in five years, I would've signed on anyway. And it's been demonstrated to be much safer than that. Unless something
other than HRT side effects kills me - accidents and illness happen regardless of HRT - chances are in my favor of having many more awesome years of not even considering suicide anymore, where for awhile there, it was a daily consideration.
As for "messing with nature" everyone who uses that as an argument against something is, in my experience, a gigantic hypocrite (do they enjoy air conditioning? Toilets? Food that they did not have to personally hunt or gather? Modern medicine?) with no rational objections to the thing at hand, so they desperately grasp at emotional straws instead.