Treatment of Riedel's thyroiditis (RT) consists of surgery in cases of local limited fibrosis. In most cases, however, it is required the use of anti-inflammatory agents like glucocorticoids or, in those who fail to respond or relapse, tamoxifen can be useful. We report a case of RT in a 55-year-old black woman associated with hypothyroidism and hypoparathyroidism. We evaluated the treatment with tamoxifen, 20 mg twice a day, for eleven months. After sixty days of therapy, patient had no compressive symptoms previously presented. However, in a follow-up of eleven months, there was little objective improvement by regular ultrasonography and computed tomography of the cervical region. Tamoxifen can be useful in RT, mainly when glucocorticoids are not indicated. The ideal duration of this therapy remains to be established.