Therefore, in the present study, we investigated the effects of polymorphisms in CYP3A4 20230G > A, CYP3A5 6986A > G, ABCB1 1236C > T, ABCB1 2677G > T/A, ABCB1 3435C > T, ABCG2 421C > A, and ABCC2 −24C > T on the mean steady-state dose-adjusted plasma trough concentration (C0) of lenvatinib in 40 patients with thyroid cancer. This evidence concerns the gene ABCG2 and thyroid cancer.