Hi
@Invisible Woman
Yes, it's difficult to sort out which issue is causing what.
With the complication of ME, I can't differentiate the ME fatigue from my hypothyroidism.
If something else is going on that causes fatigue it takes me a few days or longer to distinguish it from the ME, because I just think I'm in a crash. If other different symptoms come along then that twigs the realization there is a separate issue.
Several years pre ME and just before my hypothyroid diagnosis, I had exercise intolerance. An office job was OK, but as soon as I went and did my usual physical activity, a very few minutes in, I'd have to stop. My muscles just sort of came to a halt.
Once at the doctor's office potential signs, things like dry skin were pointed out to me. I think back then, (in the dark ages

), T4 was the test, and not TSH.
As for now with ME, my symptoms are much more complicated. I often feel like I'm hung over, and beaten up. Poisoned would also be a good description instead of hung over, for how I feel. And dragging my body through molasses. Or limp like a marionette at rest. There is also the feeling of weakness, which seems separate from the limp feeling. And, of course many of the other ME symptoms like OI, GI issues, memory problems, other cognitive problems etc., etc.
ETA: The problem of not being able to differentiate can mean other health issues come along that are just put down to ME, unless they are different. But worsening of symptoms one recognizes as ME, when they're not is a concern.