SUN-387 - Exceedingly Rare Kidney Metastasis of Follicular Thyroid Carcinoma Mimicking Primary Renal Cell Carcinoma

Background: Follicular thyroid carcinoma (FTC) metastasizes hematogenously, most commonly to lung and bone. Renal metastasis is exceedingly rare, with a reported prevalence of approximately 0.47% of FTC cases and fewer than 30 cases in the English-language literature. Renal metastases from thyroid carcinoma represent only ~3% of all metastases to the kidney. The solitary, unilateral presentation of renal FTC metastasis frequently mimics primary renal cell carcinoma (RCC), posing significant diagnostic and therapeutic challenges.

Case Presentation: A 79-year-old female with hereditary transthyretin (hATTR) amyloidosis with cardiac involvement and polyneuropathy, presented initially with progressive lower-extremity weakness and radicular pain (>25 years after benign thyroid FNA). MRI revealed T1 vertebral metastasis with cord compression and a 5 cm solitary left renal mass. Biopsies of both sites confirmed metastatic FTC, with immunohistochemistry demonstrating TTF-1 and thyroglobulin positivity, excluding RCC. Radioactive iodine ablation (150 mCi I-131, 12/2022) produced a robust response, with thyroglobulin declining from peak >10,420 ng/mL to nadir 521 ng/mL (late 2025). Serial FDG-PET/CT (2023–2025) showed stable T2 lesion, renal mass, and no new metastases, and surveillance was pursued. In 02/2026, PET demonstrated progression (enlarging hypermetabolic left renal mass, retroperitoneal adenopathy, multiple pulmonary nodules, thyroglobulin 631 ng/mL). Stereotactic body radiation therapy (SBRT) to the renal mass was initiated, with planned repeat RAI.

Discussion: Renal metastasis is more common in FTC than papillary thyroid carcinoma due to vascular invasion. Synchronous renal presentation at diagnosis occurs in only ~2% of cases. Radiographically indistinguishable from RCC, immunohistochemistry is essential to exclude thyroid-like follicular carcinoma of the kidney. RAI avidity is the most favorable prognostic factor, however, structural progression occurs in up to 56% despite biochemical response, and ~50% of distant-metastasis patients develop radioiodine-refractory disease. In this case, cardiac amyloidosis limited systemic tyrosine kinase inhibitor tolerance, necessitating locoregional strategies. SBRT as cytoreductive bridging prior to repeat radioiodine therapy represents an emerging approach with limited precedent in FTC renal metastasis literature.

Conclusion: Renal metastasis of FTC is rare and frequently mimics primary renal malignancy. Diagnosis requires immunohistochemical confirmation. Multimodal therapy including radioiodine, SBRT, and surgical intervention may optimize outcomes in patients unable to tolerate systemic therapy. This case highlights diagnostic complexity, prolonged disease latency, and evolving therapeutic strategies in metastatic FTC with significant comorbidity.

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