Dunno, but...

Maybe the immune system attacks the thyroid because it is loaded with toxic stuff like Bromine instead of Iodine and continues to send out faulty hormones...

That would be my first layman gut reaction to the first paragraph in this text:

What is Hashimoto's Thyroiditis?
Hashimoto's Thyroiditis is a type of autoimmune thyroid disease in which the immune system attacks and destroys the thyroid gland. The thyroid helps set the rate of metabolism, which is the rate at which the body uses energy. Hashimoto’s stops the gland from making enough thyroid hormones for the body to work the way it should. It is the most common thyroid disease in the U.S.