It seems like the most likely answer is that it is just a lot of different things in different people. Fatigue is a symptom in almost any and all illnesses.
A good tell is that physical effort beyond a point makes it worse, and you need to stay within an energy envelope or you get severe symptoms. All activity is physical effort not just say running. Putting the kettle on for example or refactoring a line of code.
You know that nice feeling when you feel better after you’ve been sick. You don’t feel that ever again.
This is in some way trivialising the suffering, but I find helps people understand a little what it's like. At it's worst whilst I might be able to get out of bed and use the toilet, it takes a level of forethought, planning and effort that is unreasonable.
Suffered with fatigue since stem cell transplant in 2011.
It has taken years to build back up to almost full time work without it being a boom/bust affair.
I can’t imagine the more severe cases because you need energy to manage the problem, like a catch 22.
We can test the blood for certain things, we can run other scientific tests, but for lots and lots of problems, we really don't have any scientific way to determine.