Mine is classified as idiopathic because they did every test in the book including nerve conduction studies, autoimmune studies. Monoclonal antibodies, brain scans, vitamin/iron, the whole nine yards. I’ve had Hashimoto’s for decades but it’s well managed. I have severe large fiber axonal damage. I believe my nerve damage began 40 years ago when I was struck a severe case of Epstein Bar Virus. I never recovered and it turned into ME/CFS (Chronic Fatigue Syndrome). I’ve been disabled my whole adult life. The nerve damage began literally overnight when I first got sick. But the numbness and progression started about 20 yrs ago. I’m not well enough to go out for physical therapy but there are balance exercises online I do when I’m up to it.