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SAT-597 Diagnostic and Therapeutic Challenges of Riedel's Thyroiditis

Abstract Background: Riedel’s thyroiditis is an extremely rare condition characterized by an overgrowth of inflammatory fibrosis that destroys the thyroid follicular architecture. The presentation can mimic malignancy, and due to its rarity, the diagnosis can be delayed. Experience with treatment of this condition is limited, but glucocorticoids and tamoxifen have both been shown in case reports to be effective. Clinical Case: A 68 year old woman presented with dyspnea and weakness. Her exam showed a diffusely enlarged thyroid with a hard texture. Labs showed hypothyroidism for which she was started on levothyroxine, and imaging revealed a goiter with a dominant 3.1cm left thyroid nodule causing tracheal narrowing. Laryngoscopy showed R vocal fold paralysis. FNA of this nodule was non-diagnostic; therefore a core biopsy was attempted which showed no thyroid tissue but skeletal muscle and sclerotic fibro-connective tissue infiltrated by chronic inflammatory cells. Neck CT done 1 month after initial presentation showed a diffusely enlarged and heterogeneous thyroid gland encasing the R carotid artery, infiltrating the peri-thyroidal fat, and causing tracheal narrowing. Given ongoing concern for malignancy, she had open incisional thyroid biopsy showing dense fibrous tissue with chronic inflammation and entrapped skeletal muscle, but again no thyroid tissue was identified. Pathology was believed to be consistent with Riedel’s thyroiditis. She was started on prednisone 60mg daily with improvement in her symptoms, and the steroids were gradually tapered off after 9 months of treatment given stable imaging.   The patient remained off steroids and without new symptoms for 1.5 years.  She was then admitted for syncope and had further evidence of disease progression on CT scan, with continued growth of the mediastinal component as well as encasement of the bilateral internal carotid arteries, right brachiocephalic trunk, trachea, and upper esophagus. Repeat biopsy again showed dense fibrous tissue with scattered small lymphocytes and plasma cells. She was started back on prednisone 60 mg daily, and imaging done after 3 months of treatment showed shrinkage of the size of the neck and mediastinal mass. However, the patient was noted to have Cushingoid features, including moon facies and weight gain. Therefore, tamoxifen 10mg twice a day was started, and the glucocorticoids were successfully tapered off. To date, she remains asymptomatic without evidence of disease progression while on tamoxifen 10mg twice a day. Conclusion: Riedel’s thyroiditis is a rare chronic inflammatory disease characterized by an invasive fibrotic process that can mimic malignancy. Treatment with glucocorticoids and/or tamoxifen can result in improvement in symptoms and reduction of the size of the mass.