Hashimoto's thyroiditis or chronic lymphocytic thyroiditis is an autoimmune disease where the body's own T-cells attack the cells of the thyroid. It was the first disease to be recognised as an autoimmune disease.
This disorder is believed to be the most common cause of primary hypothyroidism in North America. It occurs far more often in women than in men (10:1 to 20:1), and is most prevalent between 45 and 65 years of age. _Wikipedia