SHBG and Estrogen Effects?

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jamesac

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Hi there, in repro module right now and I'm a little confused as to how SHBG levels alter the effects on estrogen and testosterone.



B&B says that with increased SHBG, both estrogen and testosterone levels will decrease, but since the relative amount of estrogen to testosterone increases, you get greater effects of estrogen (ie. gynecomastia). But why is this the case if free estrogen is down? Why is the ratio of testosterone to estrogen important? Estrogen and testosterone don't compete for the same receptors or oppose each other's actions, at least not that I'm aware.



On a related note, in the next lecture Dr. Ryan says that anabolic steroids (exogenous testosterone) can cause gynecomastia because some is converted to estradiol. If the ratio of androgens to estrogen is what's important for their effects (as opposed to just total levels), why does this happen? I would have thought that pumping someone with a load of testosterone would inhibit breast growth if anything, or at least inhibit production of SHBG and decrease the effects of estrogen through that mechanism?



Any help is greatly appreciated because I've done quite a lot of searching online and can't find a good explanation.

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