If Tg cannot be used as a tumor marker in patients with PTC (for example, if the patients’ serum is positive for TgAb or if serum TgAb is negative but the serum level of Tg is low), it would be worthwhile to perform immunostaining for CA19-9 in the patients’ carcinoma lesions to evaluate its potential as a surrogate tumor marker. Here, TG is linked to neoplasm.