When a man uses exogenous testosterone, his body shuts down its natural production of testosterone and he has to use something to help get it going. So why when a girl uses birth control that contains exogenous estrogen does her natural production not shut down and force her to use something to get it going again?

My girlfriend's a pharmacist and she didn't know why (though we were at the bar drinking when I asked) so I've turned to AM. Anyone know?