SUN-317 Diagnostic Challenges of Follicular Thyroid Cancer With Sellar Involvement: A Rare Presentation

Abstract Disclosure: R. Villela: None. C.M. Acosta: None. A. Epstein Botwinick: None. C.V. Villabona: None. M.A. Jara: None. INTRODUCTION: Well differentiated thyroid cancers most commonly present with metastasis to lung, bone, and lymph nodes, occurring in about 4% to 15% of all cases. Brain metastasis occurs more seldom, in 0.15% to 1.3% of all well differentiated thyroid cancer cases. Exceedingly rare, with few documented cases in the literature, is metastatic follicular thyroid cancer involving the skull base presenting as a pituitary tumor. CASE PRESENTATION: We report the case of an 80-year-old male who initially developed light sensitivity and diplopia, prompting the discovery of a solid-enhancing sellar mass with complete effacement of the pituitary gland and skull base invasion. The patient had a longstanding history of “low thyroid hormone” diagnosed in his home country Mexico, for which he had been on levothyroxine for many years. However, this was not initially considered relevant to the current presentation. Further evaluation, including imaging and histopathology following surgical resection, confirmed the diagnosis of well-differentiated metastatic follicular thyroid carcinoma. Additional imaging demonstrated multiple lung nodules, a superior mediastinal mass possibly extending from the thyroid, and various osteolytic lesions. Via biopsy of one of the lung nodules, this was confirmed metastatic disease. DISCUSSION: This case highlights the importance of considering metastatic thyroid cancer in the differential diagnosis of sellar masses, even in patients with a seemingly unrelated history of thyroid disease, as its presentation may mimic primary pituitary tumors. It also emphasizes the diagnostic challenges that arise when thyroid disease is not initially connected to presenting symptoms. Recognizing atypical metastatic patterns is essential for timely diagnosis and appropriate management. Presentation: Sunday, July 13, 2025