Lenvatinib is a standard therapy for radioiodine-refractory differentiated thyroid cancer (RR-DTC). However, because of the high incidence of adverse events resulting from this treatment, it is not easy to maintain the dose intensity of lenvatinib, especially in Japanese patients. Although the prognostic impact of lenvatinib dose interruption has been reported, the target dose intensity of lenvatinib to optimize survival benefits remains unknown. We therefore propose a target dose intensity of lenvatinib during the first 8 weeks of treatment. We retrospectively analyzed 42 RR-DTC patients who were treated with lenvatinib for more than 8 weeks. We performed receiver operating characteristic curve analysis to determine the cut-off value of 8 weeks’ relative dose intensity (8w-RDI) to predict treatment response, and identified that the optimal cut-off value of 8w-RDI was 60% (sensitivity: 81.8%; specificity: 80.6%). Median progression-free survival (PFS) (not reached [NR] vs. 11.0 months; hazard ratio [HR] 0.29; 95% confidence interval [CI] 0.11–0.72; p < 0.01) and overall survival (NR vs. 27.6 months; HR 0.44; 95% CI 0.11–0.91; p = 0.03) were longer in the higher 8w-RDI (≥60%) patients than in the lower 8w-RDI (<60%) patients. Multivariate analysis revealed that 8w-RDI at ≥60% was an independent prognostic factor for PFS (HR 0.29; 95% CI 0.09–0.96; p = 0.04). Targeting for ≥60% of the relative dose intensity during the first 8 weeks of lenvatinib treatment can be sufficient to achieve significant tumor shrinkage and prolong PFS in RR-DTC patients.