Serum Adrenal Corticotropin Releasing Hormone (ACTH) is a hormone produced by your pituitary. This hormone signals the adrenals to produce more cortisol. It is a simple blood draw, done at specific times (8am or midnight) and is only complicated by its need to be drawn into a chilled tube and kept frozen. It has a super-short half life (somewhere around 14-17 minutes, depending on the source), meaning the levels present in a blood sample drop quickly at room temperature. I *always* remind the lab aid about this tests' requirements if they haven't already obviously prepared a chilled tube, etc. Depending on the lab, this test can take a bit longer than the cortisol, but it too has a fairly quick turn around. A serum ACTH level can be a very helpful test for diagnosis as it helps point to the source of the excess cortisol state endogenous hypercortisolism (ie, the problem stems from within the body).
In the case of Cushing's Disease, there is a tumor on the pituitary that produces it's own ACTH, effectively bypassing the feedback loop that would otherwise keep your levels just where you need them in a healthy body. This in turn causes your adrenals to produce cortisol your body doesn't actually need, and it sets off a huge chain of events that can cause a lot of damage to your body. In this situation it also suppresses your own pituitary's production of ACTH (there is already enough in the blood supply, so your pituitary does not call for more) with it basically going to sleep as it gets out of the habit. In cyclic cases, it is thought that the tumor turns off production (this is poorly understood and there is no explanation as to why or how, to my knowledge), and this accounts for a cyclic person swinging into a "low cycle" or normal cycle. For this reason, the pituitary of a cyclic patient doesn't always become fully suppressed and post-operative testing may not be as clear. If the pituitary can wake up and kick in, your body may not crash to a "zero cortisol state" postoperatively as is expected and watched for as a sign for remission in florid cases (though it certainly still does happen). It also may mean a lower requirement for post-op steroid replacement. But this is all so individual, dependent upon your pre-operative levels, and how your body copes with the changes.
ACTH within the normal range or somewhat elevated in the presence of hypercortisolism is suggestive of a pituitary tumor source as described above.
Low or absent ACTH levels are indicative of an adrenal source, meaning there is a tumor on an adrenal glad itself that instead of producing ACTH, produces cortisol more directly. Your pituitary senses the excess cortisol and thus does not send out ACTH to produce more. This too can lead to pituitary suppression.
An excessively high ACTH level is suggestive of what is called an ectopic source. This can be a tumor almost anywhere else in the body, that produces high amounts of ACTH. There are some specific types of imaging that can help locate these tumors, which I'll discuss briefly in a later post.
The most common endogenous source is pituitary, a small percent is adrenal, and an even smaller percent are ectopic.