It is called chronic because the inflammatory process is most often a slow one, taking place over months to years.
Chronic thyroiditis is an autoimmune disease.
Autoimmune diseases develop because of a problem in the immune system. Normally, when foreign materials such as virus or bacteria enter the body, the immune system tries to destroy them. Special cells and substances called antibodies are made to rid the body of these invaders. Autoimmune disease develops when the immune system uses cells and antibodies to attack its own body tissues rather than viruses and bacteria. The antibody to thyroid peroxidase enzyme is associated with autoimmune damage to the thyroid gland, which can lead to chronic thyroiditis.