2.2.3. Uni- and Multivariate Analysis

To analyze possible risk factors for poorer outcomes, we performed a univariate and a multivariate analysis. Of the factors regarding primary presentation of patients, age ≥ 55 years at initial presentation was significantly associated with poorer outcome (Hazard ratio, HR = 7.605, *p* = 0.006), whereas gender (HR = 10.675, *p* = 0.104), histological subtype (HR = 8.123, *p* = 0.746), and the presence of mETE (HR = 9.556, *p* = 0.256) were not associated with poorer outcome. Regarding TNM-staging, N1a/N1b-stage (HR = 7.239, *p* = 0.001) was the only unfavorable prognostic factor for poorer outcome, whereas T-stage>T1 (HR=8.837, *p*=0.251) was not associated with lower responder rates. Furthermore, a mean initial I-131-dose of ≥7400 MBq (HR = 14.028, *p* = 0.026) was associated with significantly poorer outcome in the univariate analysis. In the multivariate analysis, N1a/N1b-stage was the only independent unfavorable prognostic factor for treatment success (HR = 0.716, *p* = 0.024). Age ≥ 55 years at initial presentation (HR = 1.003, *p* = 0.405) and mean initial I-131-dose of ≥7400 MBq (HR = 0.915, *p* = 0.672) were not significantly associated with poorer outcome in the multivariate analysis. All risk factors are summarized in Table 3.


**Table 3.** Prognostic risk factors for poorer responder rates (uni-/multivariate analysis).

T, tumor; N, nodus; mETE, minimal extrathyroidal extension; GBq, gigabecquerel; HR, hazard ratio; CI, confidence interval; RAI, radioactive iodine.
