ABSTRACT: Radiation-induced thyroid dysfunction is very rarely associated with peptide receptor radionuclide therapy (PRRT) for neuroendocrine tumors using β-emitting radionuclides. With the emergence of α radionuclide such as Ac, PRRT using these radionuclides has shown better results; however, data on the toxicity profile are limited. This is a case report describing the thyroid dysfunction developed in a patient with inoperable metastatic neuroendocrine tumor with unknown primary after PRRT using Ac-DOTATATE, which has never been reported previously.