How does hashimoto's thyroiditis develop?

Variable: Hashimoto's thyroiditis develops when the immune system misclassifies one or more thyroid proteins as foreign (not belonging to that person) and begins making antibodies that target the protein(s). This can cause the thyroid to become enlarged (form a goiter), develop regional growths, or nodules, or make too little thyroid hormone. Once in a while, it can result in too much thyroid hormone.

Thyroid antibodies: The immune system makes anti-thyroperoxidase antibodies and/or anti-thyroglobulin antibodies which are both directed at different parts of the thyroid gland. These antibodies infiltrate and damage the thyroid gland painlessly, often rendering the gland underactive.