False-positive result of a total-body scan caused by benign thyroidal tissue after I-131 ablation

This case report illustrates a false-positive result of an I-131 total-body scan caused by abnormal, noncancerous thyroid tissue. A 39-year-old woman underwent an open biopsy and thyroidectomy for a papillary thyroid carcinoma. She was treated by ablation with 150.8 mCi I-131. A follow-up total-body scan revealed a solitary focus of increased activity near the midline at the upper border of the larynx that was subsequently excised. Histologic analysis indicated a fragment of thyroid tissue with chronic inflammation, fibrosis, and squamous metaplasia. No evidence of thyroid carcinoma was present. Hypofunctioning or nonfunctioning residual tissue within the thyroglossal duct may have been suppressed under euthyroid conditions, protecting it from ablation. This tissue may have become stimulated by the high thyroid-stimulating hormone levels, accumulating I-131 and producing a false-positive result of the scan.