My adrenal levels are twice the norm. But my doctor said I can take steroids to lower it. How does this work?

Good question. Significant reduction in androgen levels of adrenal source (such as dhea-s & androstenedione) if the body is subjected to exogenous steroids such as Dexamethasone due to creation of negative feeback on the pituitary gland causing decreased acth production. This in turn will reduce the signaling to the adrenal gland that will reduce overall adrenal hormone production and improved clinical symptoms.
Negative feedback. If you're talking about (for instance) too much cortisol, in some cases, a suppressive dose of a steroid will turn off your adrenal function and you will be using the replacement hormone instead of your own. It's highly complex. The endocrine system works off of negative feedback and a system of stimulating hormones and receptors and down-regulation. In this case, you're going to have to trust.