SAT-363 Riedel’s Thyroiditis Refractory to Glucocorticoids Requiring Treatment with Tamoxifen

Abstract Disclosure: J. Arkin: None. M. Khan: None. P. Guido: None. Background: Reidel’s Thyroiditis is a chronic inflammatory disease of the thyroid gland causing fibrosis, and is a manifestation of Immunoglobulin G4-Related Disease (IgG4-RD). First line treatment is glucocorticoids, however in refractory cases tamoxifen has been shown to be an effective treatment. Clinical Case: A 59 year old woman with a history of hypothyroidism presented with a dysphagia and neck pain. On physical exam, she had an enlarged and hardened thyroid gland. Thyroid ultrasound and computed tomography (CT) imaging showed a diffusely enlarged thyroid without discreet nodules and with mass effect on surrounding structures. Fine needle aspiration, and core needle biopsy were performed, and cytology were consistent with lymphocytic, fibrosing thyroiditis and she was given the diagnosis of Reidel’s Thyroiditis. The tissue had positive staining for IgG4 and the patient had elevated inflammatory markers (CRP 10.6 mg/L, reference range <8.0 mg/L). Despite elevated serum IgG4 levels of 107.2 mg/dL (reference range: 4.0 - 86.0 mg/dl), she was without other organ involvement per positron emission tomography computed tomography (PET CT). Thyroiditis was, therefore, the only manifestation of her IgG4-RD. She remained symptomatic with compressive symptoms, and initially began treatment with steroids, receiving a three-month prednisone taper with some symptomatic improvement. She was weaned off steroids and compressive symptoms returned, with repeat imaging demonstrating persistent thyromegaly, so prednisone was restarted. It was determined that non-surgical management with tamoxifen was appropriate for this patient. Her treatment plan was complicated by a lung cancer diagnosis, necessitating need for surgical intervention with nodule resection, right upper lobectomy, and mediastinal lymphadenectomy. Due to the increased risk of venous thromboembolism (VTE) associated with tamoxifen, initiation was delayed until the patient was no longer at high risk for VTE after surgical intervention and she regained significant ambulatory function. She remained on steroids for an additional three months without significant improvement in compressive symptoms. She was then transitioned to tamoxifen 20 mg daily, with improvement in symptoms and thyromegaly on repeat imaging within three months. Conclusion: This case re-demonstrates the efficacy of tamoxifen as a rapid-acting treatment for Reidel’s Thyroiditis, despite delays in initiation. Presentation: Saturday, July 12, 2025