Hi Kersten,
FNA can be a different experience for different people. Generally, though, it is not very painful and since most patients are only given a local anesthetic (lidocaine, usually), they can return to work if they feel able. I know several members have taken either Tylenol or Advil or something like that after the FNA and been fine.
Depending on where you live, it will be either your doctor or a radiologist doing the FNA. In my case, a radiologist did the FNA guided by ultrasound. It was then checked immediately by a pathologist to make sure there was an adequate sample to be analyzed. In my case, there wasn't the first time, so they had to try, try again. For me, it was quite uncomfortable, but, as I said, a lot of members have no trouble with it at all.
As far as a Hashimoto's/cancer connection, I'm afraid I don't know the answer to that. Hopefully, someone else here will.
Good luck to you tomorrow. I hope it turns out well for you!!!
I haven't had one, so I can't comment, but this will get your question off of page two and maybe you'll get some answers.