What level of estrogen is necessary to prevent osteoperosis?

[purpletomato (imported)]

I am taking 2mg / day of Estradiol Valerate, and am interested in switching to Sandrena, an inexpensive oestradiol gel. (That might also take the load off my liver, correct?). I'll be talking with an endocrinologist, but might end up ordering more meds in the meantime (appointments usually take weeks in my experience).

I got some 10 mg generic fosamax (called osteofos) anti-osteoperosis pills, and was wondering if I need to be taking them (in general, I've been pretty bad, since they need to be taken in the morning before eating).

Thanks!