Well, no one here could possibly diagnose her. Most of us aren't medical professionals, and those that are would need a lot more information.
Be that as it may, since thyroid has been apparently ruled out, I'd be looking in the direction of things like anemia and the Epstein-Barr virus. There are of course a million other possibilities but those two spring to mind immediately.
All of this, naturally, would be under the direction of her doctor giving her tests and following up with questions, etc. I'm sorry she's feeling poorly, and I hope you very soon get the answers that you seek.