Sacral Metastasis in Follicular Thyroid Carcinoma: Cytology Pitfalls and Serum Thyroglobulin Utility.

Sacral bone metastasis from primary follicular thyroid carcinoma (FTC) is rare. Most reported cases include factors indicative of thyroid origin, such as a history of treated thyroid cancer or newly identified thyroid nodules with malignant cytology. We herein report a 57-year-old woman with a metastatic sacral lesion of thyroid origin, initially misdiagnosed due to a false-negative fine-needle aspiration (FNA) cytology result of a thyroid nodule. The diagnosis was suspected based on an abnormally elevated serum thyroglobulin (sTg) level and confirmed through repeat core biopsy with thyroid-specific immunohistochemistry. This case highlights the limitations of FNA cytology in large thyroid nodules and underscores the potential role of sTg in diagnosing metastatic FTC in certain clinical scenarios. Thyroid carcinoma should be considered in the differential diagnosis of sacral metastases when the primary tumor is unknown.