Serum Cortisol Binding Globulin (CBG) is another simple blood draw. This one you don't do often, but just so your endocrinologist can get a feel for whether your CBG is high, low or normal.
This test measures the amount of cortisol in your blood that is in bound form. My understanding is that free cortisol is the active form, so it is the most important for determining hypercortisolism. Since a fair amount of the testing used gives results in "total cortisol" it is useful to rule out other causes for test elevation.
If your CBG is high, it can make your total cortisol results also come back high, even though it may not actually be problematic. One specific instance in which high CBG is known to be elevated and confuse testing is with oral birth control pills. They raise your CBG level and can skew (and invalidate) any test results while on them. For this reason it is suggested you be off oral birth control (estrogens specifically, I believe) for a minimum of 6 weeks prior to cortisol testing. And always inform your endocrinologist of all medications and supplements you are taking, including other forms of hormonal birth control, etc.