I found a nice article that tells us about the side effects of to low estrogen's
Estrogen’s Effect on Men in Muscle Building
Bodybuilders commonly assume estrogen always equals bad because it is considered a women’s hormone. While that is generally true, I believe that your body needs a certain level of estrogens for muscle gains. If you have ever taken an anti-estrogen, you may have noticed a muscle hardening effect, but it’s hard to gain a lot of muscle mass even though they boost testosterone levels significantly. Many bodybuilders on testosterone cycles have also noticed a difference in gains from using anti-estrogens , which is why some either advocate not using them or using low dosages.
Estrogen seems to probably have many different pathways it can help with muscle building. Estrogen helps with growth hormone levels, a important factor in muscle building. Estrogens also help with water retention. Retaining water helps hydrate the cells promoting protein synthesis and storage in the cells. The water retention also helps with strength gains, which should help long term muscle gains. Estrogen also has a vital role in the protein kinase cascade, which is important in the muscle building process.
While excessive Estrogen in men is certainly a bad thing, eliminating it completely through anti-estrogens isn’t going to automatically mean a large boost in gains. There are too many ways that estrogens can help your gains, that the boost in testosterone won’t fully make up for all the losses from lower estrogen.