Low thyroglobulin concentrations after thyroidectomy increase the prognostic value of undetectable thyroglobulin levels on levo-thyroxine suppressive treatment in low-risk differentiated thyroid cancer

Design: Recombinant human TSH-stimulated thyroglobulin (Tg) levels (rhTSH-Tg) are sufficient for early follow-up of low-risk differentiated thyroid cancer (DTC) patients after thyroidectomy and radioiodine (131I) remnant ablation (RAI). Serum Tg levels at the time of remnant ablation (ablation-Tg) is thought to be related with rhTSH-Tg and may be predictive of recurrent disease. During long-term follow-up, Tg levels on levo-T4 (L-T4) suppressive treatment (suppressive-Tg) is sufficiently sensitive to avoid further evaluations in patients with undetectable rhTSH-Tg. The aim of our study was to verify whether, in a subgroup of low-risk DTC patients, the association of low ablation-Tg levels (<10 μg/l) with undetectable suppressive-Tg concentrations has a sufficient negative predictive value (NPV) for recurrence of disease, leading to avoid rhTSH testing. Methods: We enrolled 169 low-risk DTC patients treated by thyroidectomy + RAI and undetectable suppressive-Tg at 12-month follow-up. In all patients, we retrospectively evaluated ablation-Tg and rhTSH-Tg. For all patients, 2-yr follow-up was available. Results: Based on rhTSH-Tg>2 μg/l, relapsing disease was histologically proven in 2 patients. rhTSH-Tg levels between 0.6–2.0 μg/l, with no evidence of disease, was observed in 10 patients (6%). One hundred and fifty-seven patients showed undetectable rhTSH-Tg. The NPV of undetectable suppressive-Tg was 92.8%. The ablation-Tg level was <10 μg/l in 140 patients. In this group, the NPV of undetectable suppressive-Tg was 100%. Conclusion: Our data indicate that undetectable suppressive-Tg value, combined with ablation-Tg levels <10 μg/l, may avoid a significant number of high-cost rhTSH-Tg test.