BACKGROUND Riedel's thyroiditis is a rare form of immunoglobulin G (IgG) 4-related invasive fibrosis of the thyroid gland; given its scarce incidence, standardized therapeutic guidelines are unavailable. Although complications are unusual, obstructive symptoms produced by the stony-hard goiter may put patients' lives at risk. The diagnosis must be biopsy-proven, and treatment consists of thyroid hormone replacement and anti-inflammatory drugs, although sometimes thyroidectomy may be required. CASE REPORT A 69-year-old woman presented with a 7-month history of progressive hypothyroidism and obstructive dysphagia. On physical examination, she had a large, stony-hard goiter. A Doppler ultrasound study revealed a massive, avascular enlargement of the thyroid gland. A computed tomography scan failed to demonstrate any extrathyroidal extension of the abnormal tissue. A Tru-Cut biopsy of the thyroid was performed. Extensive replacement of thyroid follicles by prominent bands of fibrous tissue was observed, with follicular obliteration and mild focal occlusive phlebitis. A lymphoplasmacytic infiltrate was clearly identified; no oxyphilic nor giant cells were found. On immunohistochemistry, the immunoglobulin G (IgG) 4/IgG ratio in the plasma cell infiltrate was 40%; increased serum IgG4 levels were also found, supporting the diagnosis of Riedel's thyroiditis. The patient was successfully treated with levothyroxine replacement and tamoxifen with prompt resolution of obstructive symptoms. CONCLUSIONS Fibrous thyroiditis should be considered in the differential diagnosis of primary hypothyroidism in a patient with a stony-hard goiter. Although steroids are often used as a therapeutic strategy for this disease, our patient had an excellent therapeutic response to tamoxifen, avoiding adverse effects associated with steroid therapy, the higher cost of monoclonal antibody therapy, and surgery-associated risks.