SUN-382 - Metastatic Follicular Thyroid Carcinoma with No Identifiable Intrathyroidal Primary Tumor

Background: Follicular thyroid carcinoma (FTC) typically presents with an identifiable intrathyroidal primary lesion. Presentation with metastatic disease in the absence of malignancy within the thyroid gland is rare and poses diagnostic and management challenges.

Case: An 83-year-old male presented to orthopedics for evaluation of an impending pathologic fracture of the left femur. Imaging revealed a 5.3-cm lucent lesion involving the left femoral neck. CT of the chest, abdomen, and pelvis was negative for metastatic disease except for a 1.6-cm left thyroid nodule, which was not visualized on follow-up thyroid ultrasound. The patient underwent surgical excision of the femoral lesion. Pathology demonstrated metastatic carcinoma consistent with thyroid origin, positive for thyroglobulin, TTF-1, and PAX8, and negative for BRAF mutation. A nuclear medicine bone scan showed abnormal uptake limited to the left intertrochanteric femur without additional sites of disease.
The patient subsequently underwent total thyroidectomy. Pathologic evaluation demonstrated follicular nodular disease without evidence of malignancy. Postoperative PET-CT, CT neck, and radioiodine I-123 whole-body scan showed uptake confined to the thyroid bed, with no evidence of metastatic or ectopic thyroid tissue.

Discussion: Metastatic FTC in the absence of an identifiable intrathyroidal primary tumor is uncommon. Proposed mechanisms described in prior reports include spontaneous regression of the primary tumor, missed intrathyroidal microcarcinoma, or malignancy arising from ectopic thyroid tissue. Given comprehensive thyroid pathology and imaging without evidence of malignancy, primary tumor regression is the most likely explanation in this case. Management is further complicated by the lack of radioiodine avidity of the metastatic bone lesion, limiting the utility of radioactive iodine therapy.

Conclusion: This case highlights a rare presentation of metastatic FTC with a tumor-free thyroid gland and underscores the importance of considering atypical disease biology when evaluating metastatic thyroid carcinoma. The patient is currently managed with close surveillance and serial imaging.

*Unless otherwise noted, all abstracts presented at ENDO must not be released to the press or the public until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.*