Stimulated thyroglobulin values above 5.6 ng/ml before radioactive iodine ablation treatment following levothyroxine withdrawal is associated with a 2.38-fold risk of relapse in Tg-ab negative subjects with differentiated thyroid cancer.

BACKGROUND: Serum thyroglobulin (Tg) is the key parameter used in the follow-up of subjects with differentiated thyroid cancer (DTC). Current guidelines advise its measurement to take place when Thyrotropin (TSH) levels are >30 µU/ml (stimulated Tg) and when TSH < 0.1 µU/ml (suppressed Tg). Although stimulated Tg levels <1 ng/ml have been shown to display excellent prognosis, relapses may occur despite low Tg levels. Recently, very low cut-off levels of stimulated Tg have been proposed for determining the recurrence risk in these subjects. In this study, we aimed to assess the association between ablative stimulated Tg obtained before radioactive iodine ablation therapy (RAI) (ASTg) and late stimulated Tg obtained 6-12 months after primary therapy (LSTg) in a group of subjects with DTC. We also aimed to establish a cut-off level of Tg for recurrence.

METHODS: We retrospectively analyzed 393 subjects with low or intermediate risk DTC diagnosed at our institution between January 2000 and December 2010 with a mean follow-up period of 64.4 months (range 14-192 months). All stimulated Tg levels were performed following levothyroxine withdrawal in this study.

RESULTS: Histopathological analysis indicated papillary carcinoma in 362 (92.1%) subjects and follicular carcinoma in 31 (7.9%) subjects. Three hundred and twenty-four (82.4%) of our cases were females, and 69 (17.6%) were males. Recurrence occurred in 82 (20.9%) of the subjects. Relapse was significantly more frequently observed in subjects with ASTg ≥ 2 ng/ml; and LSTg ≥ 2 ng/ml. (p = 0.004 and p < 0.001, respectively). In subjects negative for thyroglobulin antibodies (Tg-ab), an ASTg value ≥5.6 ng/ml was established to increase the risk of recurrence by 2.38-fold (p = 0.002), whereas an LSTg ≥ 0.285 ng/ml increased the risk of relapse by 3.087-fold (p < 0.001).

CONCLUSION: As a result of this study, we determined that the optimum cut-off level for both ASTg and LSTg may be lower than those previously reported. Using such a lower cut-off may improve sensitivity for detecting relapse.