Think of PH/AAS as "alien testosterone" (not from outer space, I mean not native to your body) - exogenous is the real word for it.
Any time you're introducing alien testosterone, your body sees an opportunity to save the resources it usually invests in creating its own "native" or "endogenous" testosterone. Basically, your body says "hey, I'm getting all this free Testosterone, why waste time and resources making any more?" and will actually shut down (decrease) the production of your body's native endogenous test.
Think of it this way - if every time you checked your mailbox it was full of cooked steak (okay okay, gross example but go with me), you'd probably stop cooking steak, right? josh092