Very low thyroglobulin level and rare metastatic location in radio-iodine refractory DTC should raise suspicion for initial misdiagnosis.

This paper presents a case of a 38-year-old man initially diagnosed with an oncocytic variant of follicular thyroid carcinoma (FTC) with extensive lymph node involvement. Following total thyroidectomy and radioiodine therapy, the patient developed non-iodine-avid pulmonary and choroidal metastases, despite low thyroglobulin (Tg) levels. A [99mTc]Tc-octreotide scan showed avidity in metastatic lesions, raising suspicion for an initial misdiagnosis. Subsequent evaluation confirmed elevated serum calcitonin levels, leading to the accurate diagnosis of medullary thyroid carcinoma (MTC) confirmed by a pathologic review of the initial surgical specimen. This case highlights consideration of the potential for primary misdiagnosis in radio-iodine refractory differentiated thyroid cancer (DTC) with low serum Tg levels and emphasizes the importance of a thorough pathological review in patients with atypical presentations.