Illustration showing Riedel's thyroiditis. This is a rare, chronic inflammatory disease of the thyroid gland characterised by dense fibrosis (formation of scar tissue) that replaces normal thyroid parenchyma. This causes the thyroid gland to become very hard. This can obstruct nearby organs, particularly the trachea (windpipe). Surgery may be required, but usually corticosteroid drugs are used to reduce the inflammation. It is thought to be an autoimmune disorder. The thyroid's function, secreting the metabolic hormone thyroxine, is not normally affected in Riedel's thyroiditis.