In 2014/2015, tyrosine kinase inhibitors (TKIs) were introduced as a secondary treatment for refractory differentiated thyroid cancer (DTC) in Japan. While renal dysfunction is an adverse event of TKI, data on this adverse event in TKI-treated DTC remains insufficient. Here, we investigated renal function in patients undergoing TKI treatment for DTC and evaluated the efficacy of dose reduction/withdrawal for cases of renal dysfunction.A total of 73 cases of radioactive iodine-refractory DTC treated with sorafenib (n = 22) or lenvatinib (n = 51) were included. Patient data evaluated were TKI treatment period, estimated glomerular filtration rate (eGFR) before and after TKI therapy, incidence and degree (maximum value at time of TKI treatment) of proteinuria, and albumin levels before and after TKI therapy were compared.The mean ΔeGFR was -6.75% with lenvatinib and +5.90% with sorafenib. It was not significant (P = .15). The mean Δalbumin was -8.90% and -5.85% with lenvatinib and sorafenib, respectively; there was no significant difference between the lenvatinib and sorafenib groups (P = .77). According to our program of TKI dose reduction and withdrawal, all patients except 2 with diabetes were successfully continuing treatment.Overall, the present results demonstrated that renal function is negatively affected by long-term TKI treatment for RAI-refractory DTC. However, heightened proteinuria, decreased eGFR and albumin levels, and significant but apparently reversible renal dysfunction were more frequent with lenvatinib than sorafenib.