Wow, that is so tough. First off, I'm sorry that you have to end your breastfeeding relationship under such circumstances. I'm glad that your cancer is curable!! I do know a woman who had thyroid cancer during her pregnancy and she waited out the treatment until right after birth. She does still breastfeed though....but I have no idea if she had the same treatment as you. I know she had the thyroid removed and she has to take meds everyday no-matter-what.
I'm sending you good vibes that you get this solved.
I'm sure the docs are making it sound MUCH easier than it is. I would be very upset, too. I weaned my first son at 23 months, while I was going through a miscarriage, and even at 23 months I was still a wreck over it.