Autoimmune thyroiditis is any number of autoimmune diseases of the thyroid (like Hashimoto's and Graves Disease), that is, as far as I know, unrelated to eating carbs (although autoimmune disease like Celiac disease, aka gluten allergy, are associated with thyroid problems sometimes). I wonder why your doctor said that?

I also think other things can cause hypothyroid like having too much estrogen. Hmmm... now that I think of it, I know 2 women on HRT who have low thyroid too. I wonder if that is just coincidence.