SAT-293 Concurrent Granulomatous Disease and Metastatic Thyroid Cancer: Diagnostic Challenges and Potential Relationships

Abstract Disclosure: A. Ragavan: None. J. Hoang: None. Introduction: The coexistence of granulomatous disease (GD) and metastatic thyroid cancer (TC) is rare and poses significant diagnostic challenges. This case highlights the complexities of differentiating between metastatic disease and GD, particularly in the context of atypical imaging and biochemical findings. Clinical Case: A 70-year-old male with poorly differentiated TC presented with a painful lytic lesion in the left femur, initially treated with a gamma nail. Imaging revealed additional bony metastases in the spine and pelvis. After total thyroidectomy, the patient received 6 GBq of I-131. Post-therapy imaging showed minimal iodine uptake, including at a dominant, enlarging, non-iodine-avid metastasis in the right iliac bone. Zoledronic acid was initiated for bony metastases. Ten months post-I-131, surveillance imaging identified new thoracic lymphadenopathy. FDG PET/CT demonstrated markedly avid lymphadenopathy in the mediastinum, hila, upper abdomen, and inguinal regions (SUVmax 7-12), while known bony metastases remained mildly avid. Biopsy of an inguinal node revealed granulomatous inflammation without malignancy. Concurrently, anti-thyroglobulin antibody (TgAb) levels rose significantly (4 to >400 IU/mL), while thyroglobulin (Tg) levels remained stable (<15 µg/L). Clinical Lesson(s) or Conclusion(s): This case underscores the diagnostic challenges of concurrent GD and metastatic TC. The discordance in FDG avidity between new lymphadenopathy and known bony metastases prompted further investigation, leading to the diagnosis of GD. The rising TgAb levels, despite stable Tg, suggest a potential relationship between GD and thyroid autoimmunity, complicating the assessment of TC status. The development of GD following zoledronic acid treatment raises questions about its role in triggering granulomatous inflammation, though this remains speculative. Clinicians should consider GD in TC patients with atypical imaging or biochemical findings. Further research is needed to explore the potential interplay between GD, thyroid autoimmunity, and TC. Presentation: Saturday, July 12, 2025