I'm not sure I can explain per se but I can tell her story.
She started getting ill when she was 13. She had a period of tiredness and pain. It seemed to go away after a bit so we left it.
Then when she was coming up to GCSEs she started experiencing joint pain. We spoke to the GP who said it was probably arthritis and got an NHS referral to (I think) a rheumatologist. He said it was also probably arthritis and started her on painkillers.
They didn't seem to make a difference, and she was increasingly tired and in pain. She struggled to get to sleep and we replaced her bed with an expensive one and redid her bedroom.
Then she had severe stomach pain and we took her to A and E and they said it was the painkillers and so she stopped taking them.
We saw the consultant again who said it wasn't arthritis as if it was it would have responded to the drugs and that he didn't know what it was.
She started A levels but the pain and fatigue continued to get worse. We saw a pain consultant privately and he diagnosed her with fibromyalgia and prescribed SSRIs and painkillers.
Eventually it got so bad she dropped out of school.
The private medical insurance agreed to pay for a second opinion and we took her to an expensive London consultant and told him to run every test he could think of because the insurance would pay for it.
37 page report, three things out of normal range.
The NHS referred her to the me/CFS service as she was now NEET (not in education employment or training) and part of that referral was checking thyroid which again came back out of range.
GP wouldn't do NHS referral to get it treated so we saw a private endocrinologist who started her on thyroxine.
It took a year to titrate to the right dose (at appointments every 2 weeks to start with).
Once on the thyroxine her pain and fatigue started to go. She had been pretty much bedbound for the whole day, so there was a long way to go to get back to normal.
After a couple of years of thyroxine she was well enough to do a part time college course, and then an access to uni and she is now at uni studying physics (she is very bright).
The pain and fatigue have lessened over the years but they are very much still there. She can't walk far and she struggles to concentrate for more than a few hours each day,