Lenvatinib, a multi-tyrosine kinase inhibitor, has been proven to be an effective treatment option for patients with iodine-131-refractory thyroid cancer. Many adverse effects of lenvatinib have been reported; thus, dose reduction is common. However, a few studies have analyzed the causes of lenvatinib dose reduction in daily clinical practice. Here, we investigate the factors involved in early lenvatinib dose reduction to analyze lenvatinib dose modification. We analyzed 20 thyroid cancer patients who began receiving lenvatinib at the Kumamoto University Hospital Cancer Center from July 2015 to November 2016. Patients were classified into the following groups based on the time until first withdrawal or dose reduction in lenvatinib: group A (early, ≤ 10 days) and group B (other, > 10 days). Patients’ clinical features and reasons for withdrawal or dose reduction were analyzed. The age range of patients was 54–91 years, and the median observation period was 293 days. There were no significant differences in the administered line of lenvatinib; the presence/absence of primary residual tumors; or the history of hypertension, proteinuria, and diarrhea between the two groups (A, n = 7; B, n = 13). The cause for initial withdrawal or dose reduction was grade 3 hypertension in all group A patients, which was significantly higher than that in group B (p = 0.0001). Our results suggest that early blood pressure control may be effective as a method to maintain the lenvatinib dose intensity.