Since 2015, the cancer treatment lenvatinib has been used for patients with advanced radioactive iodine- (RAI-) refractory thyroid differentiated cancer; however, the drug's long-term effects have not been fully investigated. We report three cases in which lenvatinib treatment initially improved the patients' conditions, although they all died approximately 2 months after leukocytosis due to very aggressive disease progression with anaplastic thyroid carcinoma transformation. Serum interleukin-6 (IL-6) was elevated in all three cases, and granulocyte-colony stimulating factor (G-CSF) was elevated in two cases. The patients had a similar clinical course, with multiorgan metastasis and aggressive disease progression. Even with advanced cancer, lenvatinib has provided control of the disease. However, as long-term use of lenvatinib grows, it is possible that similar cases will increase, and we report our findings as an alert to other clinicians.