Background 

Follicular thyroid carcinoma (FTC) commonly metastasizes, with the lungs among the most frequent sites of distant spread. Pulmonary metastases are usually identified after diagnosis of the primary thyroid malignancy and may retain radioiodine avidity. This case highlights the diagnostic challenge of pulmonary metastases preceding FTC diagnosis and subsequent identification of radioiodine-refractory disease. 

Case Presentation 

A 70-year-old woman with a remote history of toxic thyroid nodules treated with two courses of radioiodine therapy subsequently developed hypothyroidism requiring levothyroxine replacement. Her medical history included presumed pulmonary sarcoidosis, chronic obstructive pulmonary disease, and a 20-pack-year smoking history. Serial chest imaging demonstrated enlarging bilateral FDG-avid pulmonary nodules. CT-guided biopsy revealed tumor cells positive for TTF-1 and PAX8, with weak chromogranin expression, focal weak thyroglobulin staining, and a Ki-67 proliferation index of approximately 10%, raising suspicion for metastatic thyroid carcinoma. Serum thyroglobulin was markedly elevated (188 µg/L) with negative thyroglobulin antibodies. Thyroid ultrasound identified a highly suspicious TR5 nodule, and total thyroidectomy confirmed follicular thyroid carcinoma in the left lobe. Following thyroid hormone withdrawal, the patient underwent radioiodine ablation with 4000 MBq I-131. Post-treatment imaging demonstrated uptake confined to the thyroid bed, with no uptake in the pulmonary lesions. Follow-up CT demonstrated continued progression of the pulmonary nodules. I-124 PET/CT after prolonged levothyroxine withdrawal again demonstrated absent radioiodine uptake within the pulmonary metastases. The thyroid multidisciplinary team diagnosed radioiodine-refractory metastatic FTC and recommended repeat biopsy for comprehensive molecular profiling (CaSP panel). 

Discussion 

This case demonstrates the diagnostic complexity of metastatic FTC presenting progressive pulmonary nodules in the setting of presumed sarcoidosis and previous radioiodine exposure. Integration of serum thyroglobulin, thyroid-specific immunohistochemistry, advanced radioiodine imaging, and multidisciplinary review enabled diagnosis. Absent uptake on both I-131 and I-124 imaging confirmed radioiodine-refractory disease and supported molecular profiling to guide future targeted therapy.