Riedel's thyroiditis is a rare form of chronic thyroiditis characterized by extensive fibrosis of the thyroid gland, also affecting adjacent tissue. Riedel's thyroiditis clinically resembles anaplastic thyroid or lymphoma, with a woody goiter attached to adjacent structures, and compressive. Findings from scintigraphy, ultrasonography, and cytology are rarely conclusive. A surgical biopsy is generally required to rule out a malignant lesion before the diagnosis of Riedel's thyroiditis can be confirmed. The most useful imaging method is computed tomography of the cervical mediastinal region, recommended during the diagnostic process and the follow-up. A thyroidectomy or dissection of the neck is in theory not recommended. Because the pathogenesis of the disease is not yet understood, the therapeutic approach remains empirical and imperfect. Corticosteroids are effective in early stages of the disease, but controversial thereafter. A dose of 10-20mg per day for 4-6 months is recommended, possibly continued thereafter if effective. Tamoxifen is a good alternative at 20 x 2 mg per day, to be reduced if adverse effects occur to 10 x 2 mg per day. There is not yet any consensus about the role of anti-TNFalpha agents and other immunosuppressive treatment for either local or more general forms of multifocal fibrosclerotic diseases.