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Summary objective This study was designed to investigate whether an elevated serum antithyroglobulin antibody (TgAb) reflects cancer recurrence in thyroglobulin (Tg)-undetectable patients with differentiated thyroid carcinoma (DTC) after thyroid ablation. design We measured serum TgAb level and evaluated the disease status in 226 DTC patients who had undergone remnant ablation and showed an undetectable Tg result as assessed by immunoradiometric assay. measurements Radioligand assay of TgAb was performed. Recurrence was assessed by 131 I scan, 18 F-fluorodeoxyglucose positron emission tomography, sonography, computed tomography, or by surgical operation. results Fifty-one patients (22·6%) of the Tg-undetectable patients showed positive TgAb, and 25 (49·0%) of these were confirmed with recurrence. The recurrence rate of TgAb-positive patients was higher than that of TgAb-negative patients (3·4%; P < 0·0001). During follow-up, 73·1% of the disease-free patients showed spontaneously decreased TgAb levels. A total of 71·4% of patients with recurrent cancer, who showed responses to surgical operation or radio-iodine treatment, also showed a decreased TgAb level. conclusions Persistently elevated TgAb levels appear to serve as a useful marker for recurrent or persistent DTC in patients with undetectable serum Tg results. Thus, the routine measurement of TgAb in such patient populations may be indicated. [ABSTRACT FROM AUTHOR]