Riedel thyroiditis is a form of chronic thyroiditis in which thyroid parenchyma is replaced by dense fibrous tissue. It is related to a group of idiopathic systemic disorders known as inflammatory fibrosclerosis and is associated with an increased number of IgG4-positive plasma cells. Riedel thyroiditis has also been referred to by other names such as Riedel struma, fibrous thyroiditis, and invasive thyroiditis. Dense fibrosis may engulf vital structures in the neck such as trachea causing severe dyspnea. Clinically, it may mimic carcinoma. This low-power image shows hyalinized fibrous tissue with inflammatory cells completely replacing the affected part of the thyroid gland.