Thyroglobulin antibodies in differentiated thyroid cancer

A retrospective review of patients with differentiated thyroid cancer (DTC) who were seen between 1984 and 1996 at the Royal Marsden Hospital identifi ed 40 patients with serum thyroglobulin antibodies (TgAb). These antibodies can interfere with the immunoradiometric assay for serum thyroglobulin (Tg ) used at this hospital, with resulting underestimation of the Tg level. A review of the case notes was carried out to ascertain the clinical signific ance of TgAb. The median follow-up from diagnosis of DTC was 26 months (ran ge 3-401). The median age at diagnosis of DTC was 50 years (range 13-83). Patients were grouped according to the TgAb titre (high titre: TgAb >1/100, n = 28; low titre TgAb <1/100, n = 12). Thirteen patients relapsed, 11 in the high titre group and two in the low titre group. Sites of recurrence we re: neck (12 = 9), lung (n = 5), bone (n = 4) and brain (n = 2). No patient in the high titre group showed an elevated Tg with recurrence. One patient in the low titre group showed a Tg response to recurrence. Overall, the Tg assay failed to detect 92% of recurrences. Eight patients in the high titr e group developed TgAb, apparently in response to tumour progression. In a third patient in the low titre group, the TgAb also acted as a 'tumour mark er'. Thus, overall TgAb acted as a tumour marker in nine of the 40 (22.5%) patients in whom it was detected, and in nine of the 470 (1.9%) patients on follow-up during this time period. The overall survival of the whale group was 69% at 10 years. For patients with papillary carcinoma (n = 34) overal l survival was 78% at 10 years. Laboratories should report routinely the presence of TgAb, with a caution i ndicating the direction of possible error (which depends on the assay used) . Clinicians should appreciate that Tg measurements are unreliable in the p resence of TgAb and that the development of TgAb can indicate active tumour .