Sequential Changes of Serum Antithyroglobulin Antibody Levels Are a Good Predictor of Disease Activity in Thyroglobulin-Negative Patients with Papillary Thyroid Carcinoma

Objective: To investigate whether elevated and sequential changes in serum antithyroglobulin antibody (TgAb) levels are indicators of recurrence or persistence of papillary thyroid cancer (PTC) in patients with undetectable thyroglobulin. Methods: In 56 patients followed for more than 7 years, we recorded all serum TgAb levels (except the ones determined within one year after I-131 therapy or diagnostic scans) and evaluated their disease status. All patients had undergone total thyroidectomy and remnant ablation by I-131, and they were positive for TgAb and had undetectable thyroglobulin during follow-up. The sequential changes of TgAb were defined as persistently high, increasing, persistently medium, decreasing, and decreasing to negative. Recurrence or persistence of PTC was defined as active disease as assessed by I-131 scanning, F-18-fluorodeoxyglucose positron emission tomography, ultrasonography, computed tomography, or surgical examination. Results: Of the 56 patients enrolled, 10 patients had persistent PTC and 12 patients had recurrent PTC at more than 1 year after total thyroidectomy and I-131 therapy. TgAb was persistently high in 4 patients (3 with active PTC), increasing in 6 patients (4 with active PTC), persistently medium in 16 patients (12 with active PTC), decreasing in 5 patients (none with active PTC), and decreased to negative in 25 patients (3 with active PTC). According to the trend, the patients with persistently high TgAb, increasing TgAb, and persistently medium TgAb had active disease more often (p<0.001). In the multivariable regression analyses, the trend of TgAb change was a strong predictor of PTC activity (p<0.001, R-2=-0.501). The most common diagnostic procedures performed for active disease were neck ultrasonography (21 patients) followed by F-18-fluorodeoxyglucose positron emission tomography (11 patients). The patients with autoimmune thyroid disease had better prognoses than did the patients without autoimmune thyroid disease (18% active PTC vs. 53% active PTC, p=0.02). Conclusion: The presence of TgAb is indicative of an active tumor. Sequential TgAb change is a good predictor of disease prognosis and is helpful for clinical decision making.