"Treatment surveillance of chronic glucocorticoid replacement is mainly based on clinical grounds because no objective assessment has proven to be reliable for monitoring replacement quality. ACTH cannot be used as a criterion for glucocorticoid dose adjustment, since in primary adrenal insufficiency it is invariably high before the morning dose and rapidly declines with increasing cortisol concentrations after glucocorticoid ingestion.122,124 Aiming at morning ACTH values continuously within the normal range would, therefore, lead to chronic overreplacement."
Arlt, W., & Allolio, B. (2003). Adrenal Insufficiency. Lancet, 361, 1888
It is clear in the adrenal insufficiency literature that cortisol ranges are useless for determining a patient's cortisol replacement needs yet uneducated, poorly read doctors continue to ask their patients to submit and pay for cortisol and ACTH testing to determine the patient's replacement needs. There are no reference ranges for cortisol replacement! How can any determinations be safely made by your physician if there is no data?
A doctor will ask you to get a morning cortisol test and sometimes ACTH. Did he tell you how much HC to take? Did he tell you when to take it? Does he have literature to determine what the proper numbers should be based on dosage of HC and timing of the dose (the answer to this is NO because it does not exist. If you have access to this literature and I am wrong, PLEASE post a link in the comments. I want to be wrong about this.)
The way you feel and your symptoms have to determine cortisol replacement. Doctors constantly want us to take lower and lower doses of HC but the medical literature says replacement needs to be based on "clinical grounds". The Arlt and Allolio quote is just one of many reinforcing this position.
A well meaning doctor might ask you to pay for an ACTH serum test to help you determine your cortisol replacement. He's misinformed. ACTH is a poor indicator of cortisol replacement and getting this test done can actually harm you if the results are interpreted incorrectly...and they will be interpreted incorrectly. Your doctor might suggest you start taking less HC or more without taking your "clinical grounds". Duh! Are you constantly nauseous, have diarrhea, hypoglycemia and/or low blood pressure but your ACTH draw is low? Does your doctor want to lower your HC dose based on the ACTH result but not want to pay attention to the fact that 1) ACTH can't be used to determine HC replacement 2) it has to be done under specific lab conditions and it probably wasn't 3) it has to be done first thing in the morning 4) ACTH is pulsitile and fragile so even under the best of circumstances, it can still have an abnormally low result.
Enough about the futility of the meaninglessness of ACTH results. On to the dangerous misinterpretation of cortisol testing in the undiagnosed.
Doctors will often randomly test cortisol in the undiagnosed yet have NO CLUE as to what the results should be. They look and see if the lab flags the result as "High" or "Low". This is one of the most misleading and dangerous things an uneducated doctor can do. The range is usually 2-20 (give or take, depending upon the lab). Anything that falls within that range is considered "normal".
Let's look at the "normal" range more closely.
A morning cortisol (8 am) should be somewhere around 20 with the person being asymptomatic for adrenal insufficiency. An 8 am cortisol of 15 or lower (in someone who is undiagnosed) is a reason to run an ACTH stim test. Let's just use a little common sense here. A decent morning cortisol should be 16-20ish based on the fact that a cortisol of 15 or lower is grounds to run an ACTH stim test.
A midnight cortisol should be 2-4.
If you have an 8 am cortisol of 12 and a doctor who doesn't know that the cortisol reference range is 2 at midnight and 20 at 8 am. You're screwed and dismissed as normal despite all of your other symptoms that correspond with adrenal insufficiency. You are a victim of the "normal" reference range.
Get copies of all of your labs. Study them. Learn what they mean. Search out health forums (mine is free) and get help. I can provide assistance and guidance if you want to pay for it. If you suspect you have adrenal insufficiency, you are probably right. It's hard to convince a physician who only sees "normal" numbers. All of the numbers need to be taken into account, so do symptoms and past labs.