Undetectable pre-ablation thyroglobulin levels in patients with differentiated thyroid cancer: it is not always what it seems.

OBJECTIVE To establish the frequency of U Tg (undetectable pre-ablation thyroglobulin) in TgAb- negative patients and to evaluate the outcome in the follow-up. SUBJECTS AND METHODS We retrospectively reviewed 335 patients' records. Twenty eight patients (9%) had U Tg. Mean follow-up was 42 ± 38 months. All subjects had undergone total thyroidectomy, and lymph nodes were positive in 13 (46%) patients. Tg and TgAb levels were measured 4 weeks after surgery by IMA technology in hypothyroid state. No evidence of disease (NED) status was defined as undetectable (< 1 ng/mL) stimulated Tg and negative Tg-Ab and/or negative WBS, together with normal imaging studies. RESULTS Seventeen patients (61%) were considered with NED. Four patients (14%) had persistent disease (mediastinum, n = 1, lung n = 2, unknown n = 1), and 7 (25%) had detectable TgAb by other method during their follow-up. CONCLUSIONS U Tg levels usually is associated to a complete surgery. However, in a low percentage of patients, this may be related to false negative Tg or TgAb measurement.