Here's how I have come to understand it:
Low cortisol is like high cortisol because high cortisol can make cells resistant to all hormones, including cortisol itself. This means with high cortisol, you get some resistance at the cellular level, which would theoretically make it feel allot like low cortisol. High cortisol can also cause cellular resistance to other hormones such as testosterone, progesterone, and estrogen. Cortisol is needed for thyroid hormone to be accessed by the cell. Too little and it makes you hypothyroid. Too much and it again, makes you hypothyroid. Hypothyroid is probably what causes most of the symptoms, but the cells would theoretically also be deficient in all hormones. This is why high cortisol can feel allot like low cortisol.
That is my current understanding. It has taken allot of time to understand it.
The question is, why is cortisol high or low?
It's important to get cortisol testing done. You can do the 4x/day saliva test, blood serum total and free cortisol, and also 24 hour urine collection for total, metabolized, and free (google DUTCH hormone testing for this one).