If you are interested in discussing the pros and cons of the various ME/CFS scales, I'd suggest starting a thread on it.
I'm really not. They're pseudoscience.
As for the status of immune system, endocrine system and mitochondrial functioning, we know next to nothing about these in ME/CFS, so nothing can be reliably said.
Oh, actually, there is quite a lot of research on all of these areas in ME/CFS. We actually know quite a lot.
I can't say what happens with functional medicine there in the UK, but where I live, functional medicine MDs, ARNPs, NDs, DCs, and DOs use very standard national label tests as well as done well regarded specialty labs to determine what is wrong and to decide a treatment plan. In fact, I have provided you a good deal if scientific information about this.
This is an overview of functional medicine, which is systems-oriented personalized medicine as described here, which looks into identifying and treating root causes of chronic diseases, including ME/CFS:
https://usermanual.wiki/Document/IFMWhitepaper21stCenturyMedicine.279979973.
I am just pointing out that with all the ME/CFS patients on this forum, when I ask them where they fit on the mild, moderate and severe scale, they have no trouble in telling me.
These descriptions are not the same - they day the people may work, may not work or don't work, that they are independent, dependent, and housebound or not housebound., have reduced mobility, but can do chores, or not..
Those with moderate ME/CFS are generally not able to work, probably don't leave the house much, have to perform domestic chores slowly with breaks and rests, and may need 1 or 2 hour's naps in the middle of day.
People with moderate ME/CFS have reduced mobility and are restricted in all activities of daily living, although they may have peaks and troughs in their level of symptoms and ability to do activities. They have usually stopped work, school or college and need rest periods, often sleeping in the afternoon for 1 or 2 hours. Their sleep at night is generally poor quality and disturbed.
Moderate – you may have reduced mobility, and your symptoms can vary. You may also have disturbed sleep patterns, and sleep in the afternoon.
Moderate: the patient has reduced mobility and is restricted in all activities of daily living. They have usually stopped work or education. There is poor sleep quality and duration.
Moderate – the person is mostly housebound
70 — Independent; not able to work. Cares for self; Unable to carry on normal activity.
60 — Disabled; dependent. Requires occasional assistance; cares for most needs
At my worst, I was totally brain fogged, slept 16 hours a day, had disturbed sleep at night, cooked dinner half the time, did no other housework, did a job from home or the doctors office on a computer 5 hours a week, and exercised in 1-2 minute increments, and needed significant help with moving and doing anything, and spent most of the time at home and a lot of time on the floor, pavement, grass, or whatever surface covered the ground, napping or resting wherever I happened to collapse. I had immunodeficiciency, 7 infections, 4 autoimmune problems, adrenal insufficiency, thyroid and other hormone dysfunction, half of normal mito content, mito complex I operating at 31% of normal, and had significant arsenic and mold toxicity. I don't know which category above best fits, but I don't think it was mild.
And today, though I'm 95% better, none of your definitions for mild fit at all, even though I'm not cured, still have some pretty abnormal lab values, do an extraordinary amount to support my high level of function, and can still trigger PEM with a certain level of exertion.
So, I find those scales pretty darn useless.