What's important is not how much cortisol levels a person has in their circulating blood stream or in their urine, or even, to a certain extent, how much cortisol is made by the adrenal gland. What's important, is how much cortisol gets bound to the receptor and is available for the cell to function.
Paradoxically, in post-traumatic stress disorder - even though cortisol levels are not high, there's no evidence that the adrenal makes more cortisol, for example, compared to persons who don't have post traumatic stress disorder - the cells in the body do appear to be more exquisitely sensitive to the effects of cortisol. The cortisol receptors are more sensitive, for example, more responsive to the administration of cortisol.
So, what might be happening in PTSD is that people are more responsive to stress hormones, which makes sense. If you're in a situation where you were once very afraid, it might not take that great a situation to make you that afraid again. Or, you are hyper-alert to the possibility that you might need to have a stress response, which is essentially a good thing, because it helps you adapt to the short term needs of a situation.