I understand wanting to listen to your dr. and trust their judgement. Addison's is so rare that most Drs. have never treated a patient with it.
I went to one of the top Endos. in Texas who has written articles on Addison's and participated in research...I thought I could trust his judgement, right? Wrong. He took one look at me and said I did not have the hyperpigmentation (darkening) of the skin and thus it was impossible for me to have addison's. #1 - 10% of Addison's patients never have hyperpigmentation, #2 - 3 months after I started Addison treatment with different doctor I was 'white as a sheet' so I did have darkening of skin it just wasn't as pronounced as some. My point is that not all doctors understand adrenal problems. in fact most don't.
Really there is NO reason to torture a person by tapering them off of steroids to do an ACTH stim test. The dexamethasone does not affect the test like cortef. So if you take dex for 3 weeks you can get accurate results without months of torture. My heart really goes out to you, because going down on the steroids if you need them is one of the most horrible things to go through!
You mention a normal AM plasma cortisol draw...this is not uncommon in adrenal problems. What happens many times is that the adrenals make some cortisol over night and thus have a "normal" amount of cortisol in the AM, but if you test noon & PM it will have declined to almost nothing! So as the day goes on fatigue, depression, hopelessness, hypoglycemia, achiness, muscle cramps, headaches, nausea, etc. all gets worse.
Cushings can cause fatigue so it is definitely something you want to rule out.
Waking at 3-4AM is very common for those with adrenal problems, but very, very few doctors know about this. The reason that you wake at that time is that your body's natural rhythm of making cortisol is to increase production around 1-2am until it is at it's highest amount for the day at 7-8am. So when your adrenals are damaged and unable to produce cortisol in the early morning hours it wakes you up.
Often people who wake up from adrenal problems:
-wake at the same time every night
-wake with accelerated heart rate &/or low BP
-wake with low blood sugar
-wake wide awake and have trouble going back to sleep
-May feel nauseas, achy all over
-Have muscle/joint pain
When you wake with low cortisol you often wake up with an adrenaline rush, because your body is trying to compensate for the lack of cortisol.
I suggest that you get a saliva cortisol test done, then you will know what your cortisol is throughout the day.
Can you ask your doctor about taking dexamethasone to do the stim test?
I am so sorry you are going through this. So many of your symptoms sound like low cortisol it really concerns me to think of you going lower on the steroids. It can be very dangerous if you do have adrenal insuffiency.
Has your doctor told you what the signs of adrenal crisis are so that you know if you are in trouble and need to go to hospital?