I know the HPA axis has been an interesting line of investigation, but not everyone with CFS is showing up with HPA axis issues. I still disagree with the direction that the illness is primarily a mental illness simply because it can result in some mental symptoms. If primarily it is physical in nature, as measured on various tests, and in your case may be caused by an endocrinological dysfunction, and secondarily it causes changes in mood or behavior that doesn't make it a mental illness. (I'm familiar with PEM myself but do not have any severe mood change during it myself.)

Take for example African sleeping sickness. After initial infection the patients have a number of mood and behavior changes, excessive sleepiness, confusion, etc. that might appear mental. But it would be absurd to classify it as a mental illness. It sounds very similar to the logic people are applying to ME/CFS in trying to label it mental.