Fetal and neonatal hyperthyroidism are rare, but severe, conditions caused by transplacental passage of thyrotropin receptor antibodies from a pregnant patient with current or past Graves disease. We present the case of a late preterm infant who developed thyrotoxicosis secondary to maternal Graves disease treated with radioactive iodine, followed by a review of the literature, clinical implications, and best practices on the diagnosis and treatment of neonatal hyperthyroidism. We highlight the importance of finding the underlying cause for hypothyroidism during pregnancy, ensuring those with past Graves disease receive antibody monitoring, and neonatal thyroid function is assessed within the first few days of life, so timely treatment can be initiated if necessary.