For starters, all cow’s milk naturally contains estrogen (and other hormones), so semantically speaking hormone free milk doesn’t exist. The “hormone free milk” label you commonly see on dairy products is usually referring to rBST a synthetic hormone administered to stimulate milk production. However, the reality is, rBST is banned in Canada, so in actuality all Canadian milk could potentially say “rBST free milk”.
But the story doesn’t end there. Even though the estrogen in milk is naturally occurring, its presence is worrisome because the amount of estrogen in modern milk is much higher than in previous generations. This is because traditional herding societies use tactics to minimize the amount of estrogen in their milk—namely, they refrain from milking cows during the late stages of their pregnancy, when they produce 33 times more estrogen—meanwhile presently in North America, most cows are milked 300 days a year. As a result, they’re being milked throughout the cow’s pregnancy, leading to potentially higher estrogen levels. This is concerning because it may be a factor contributing to the rise of estrogen related cancers, including breast, uterine, endometrial and testicular cancer.
I learned about this fact from: http://www.news.harvard.edu/gazette/2006/12.07/11-dairy.html