Which of the following is true about Riedel's thyroiditis -

Ans. is 'd' i.e. All the above Riedel thyroiditiso A rare disorder characterized by extensive fibrosis involving the thyroid and contiguous neck structures,o The presence of a hard and fixed thyroid mass clinically simulates a thyroid carcinoma.o It may be associated with fibrosis in other sites in the body, such as the retroperitoneum, and appears to be another manifestation of a systemic autoimmune IgG4-related disease, which is associated with fibrosis and tissue infiltration by plasma cells producing IgG4.