I've had the thyroid issue for about 10 years or so. I don't know if there is any way of finding out what caused it. It just came up one day I guess that my thyroid blood values weren't great. Wasn't experiencing any symptoms (tiredness or anything else), as far as I can remember.
Me taking my thyroid medication (L-Thyrox(ine)) has made my blood levels be totally fine for many years, I don't have (m)any other symptoms that I would know of, except for a now rather mild skulp skin issue. Not a lot more I can do about it at this time, I guess. If it is causing my EC is just speculation (although it may very well be the case).