Hashimoto's disease is an autoimmune disorder. The immune system creates antibodies that attack thyroid cells as if they were bacteria, viruses or some other foreign body. The immune system wrongly enlists disease-fighting agents that damage cells and lead to cell death. What causes the immune system to attack thyroid cells is not clear. The onset of disease may be related to: Genetic factors Environmental triggers, such as infection, stress or radiation exposure Interactions between environmental and genetic factors