Dutasteride is a dihydrotestosterone (DHT) blocker. DHT comes from testosterone, is more potent than it and is the cause of male pattern balding. However it's in pretty small amounts compared to testosterone, which causes, I think, most of the masculinization.
Dutasteride is normally a DHT blocker, and that's what it's used for, as far as I know. However, maybe it does block testosterone too. I don't know. But if it does, then why does every case I've heard of combine it with a more usual antiandrogen?
Normally, to block testosterone, you're going to use either cyproterone acetate (very potent but also slightly more risky) or spironolactone (a bit less potent, but I think it's less risky). Cyproterone acetate is basically used everywhere in the world minus in the US. The US haven't approved the product for some reason, so Americans are prescribed spironolactone instead.
Then, if male pattern baldness is a serious issue (and there are probably other reasons for using it, I dunno), sometimes doctors will supplement the regimen with additional finasteride or dutasteride.
Now all of this is only what I've read about people's hormone regimens. To be honest I hardly know what I'm talking about. But this I know: look at your blood levels in your next blood test. If it's good, it's good. If not, then perhaps you have something to worry about.
Also, weird sudden returns of libido after a while on HRT do happen. I can't explain it, but mine did somewhat come back after over a year of near-zero HRT.
