Hashimoto's disease is thought to be an autoimmune disease where antibodies destroy the thyroid.
The identity of an antibody isn't as clear as people assume. There is a cross-reactivity between thyroid antibodies and joint antibodies. People with thyroiditis and hypothyroidism often have arthritis and joint problems.
In experiments with rabbits, they showed an injury triggers an immune reaction to a cartilage implant from a cow, but if the cartilage is implanted without an injury it is not rejected.