ABSTRACT: Objective: The objective of this report is to emphasize the importance of considering thyroid cancer in the differential diagnosis, when the origin of a metastatic boney lesion is indeterminate. Methods: Diagnostic studies performed included a thyroid function test, an ultrasound, and a computed tomography (CT) scan of the neck, biopsies of the bone, and thyroid lesions. Results: A 61-year-old man was found to have incidental sclerotic bone lesions in the lumbar region on CT scan performed in the setting of a prostate abscess induced sepsis. The bone biopsy suggested metastatic follicular thyroid carcinoma. Imaging studies of the neck showed markedly enlarged left greater than right thyroid nodules. A surgical specimen from the staged total thyroidectomy showed no evidence of thyroid malignancy, despite a thorough review of microscopic tissue sections at 5 μm. A whole body scan 2-months after radioactive iodine therapy demonstrated persistent uptake in the metastatic lesion at L4 and interval progression of widely metastatic disease. Conclusion: Metastatic thyroid cancer may be present without a histopathologic evidence of thyroid malignancy, albeit rarely. When the origin of a metastatic boney lesion is unclear, thyroid cancer should be included in the differential diagnosis. Abbreviations: CT computed tomography RAI radioactive iodine Tg thyroglobulin
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