Abstract: Background and purpose: Negative pre-ablative stimulated thyroglobulin (ps-Tg) with negative ¹³¹I scan indicates disease-free survival of patients with differentiated thyroid cancer (DTC). However, negative ps-Tg with lymph node metastasis detected by ¹³¹I scan often shows in clinic. The purpose of this study was to investigate the prognosis of postoperative DTC patients with lymph node metastasis and negative ps-Tg and its influencing factors. Methods: From May 2015 to Jan. 2018, 130 DTC patients with lymph node metastasis who underwent ¹³¹I treatment for the first time in the Affiliated Hospital of Qingdao University were followed up for 6-36 months. According to the clinical outcome, they were divided into 3 groups: excellent response (ER), indeterminate response (IDR) and structural incomplete response (SIR). The gender, age, size of primary tumor, extraglandular infiltration, T stage, N stage, lymph node metastasis rate, recurrence risk stratification, ps-Tg and thyroglobulin antibody (TgAb) were compared among the three groups. Subgroup analysis of significant factors was further carried out. Results: There were significant differences in N stage (χ²=11.274, P=0.024), ps-Tg (H=9.579, P=0.008) and TgAb (H=11.632, P=0.003) among groups. There was no significant difference in gender (χ²=0.559, P=0.756), age (F=0.408, P=0.666), primary tumor size (H=1.834, P=0.400), extraglandular infiltration (χ²=1.345, P=0.510), T stage (χ²=4.494, P=0.610), lymph node metastasis rate (H=3.358, P=0.187), recurrence risk stratification (χ²=3.008, P=0.556) and dose of radioiodine (H=1.335, P=0.513). 100% (14/14) reached ER in N0 group. 77.78% (14/18) reached ER, and 22.22% (4/18) reached IDR in N_1a group. 63.26% (62/98) reached ER, 18.37% (18/98) reached IDR, and 18.37% (18/98) reached SIR in N_1b group. The median ps-Tg level in IDR group was 1.85 ng/mL, significantly higher than that in ER group (t=2.976, P=0.003) and SIR group (t=2.468, P=0.014). The median TgAb level in SIR group was 713.1 U/mL, significantly higher than that in ER group (40.42 U/mL, t=3.409, P=0.001) and IDR group (39.02 U/mL, t=2.381, P=0.017). Conclusion: N stage, ps-Tg and TgAb levels can be used as sensitive indicators for predicting clinical outcomes in DTC patients with negative ps-Tg and lymph node metastasis after the first ¹³¹I treatment. The prognosis of patients with N₀ stage is better, and those with N_1b, low ps-Tg level but significantly elevated TgAb level are more likely to have poor treatment response.

Key words: Differentiated thyroid carcinoma, Lymph node metastasis, Thyroglobulin, ¹³¹I therapy