Arimidex should have no significant effect on estrogen levels in pre-menopausal women. Arimidex (anastrozole; the same course of action use also letrozole, formestane and exemestane) blocks the conversion of testosterone to estrogen by the enzyme aromatase. Pre-menopausal women convert testosteron into estrogen directly in the ovaries and release estrogen directly into the bloodstream.
Arimidex is only suitable to combat estrogen sensitive breast cancer in post-menopausal women. Their ovaries don't make estrogen anymore and they only produce some through aromatase from testosteron (little is made e.g. in the adrenal gland) - like men do all their life.
Pre-menopausal women with estrogen sensitive breast cancer do either need to have their ovaries removed or use selective estrogen receptor modulators (SERMs) like Nolvadex or Raloxifen.
Both drug groups can be used together, because even the most potent aromatase inhibtor exemestane blocks only up to ~95% of estrogen production. Some tumors even grow with that little amount of estrogen.
If pre-menopausal women report fat loss with using Arimidex, i would deny any role of that drug, but think that it is due to better diet and exercise.
To suppress estrogen production in pre-menopausal women, LHRH-agonists like Zoladex are needed.