Hashimoto's encephalopathy (HE) is a rare condition in the childhood. It can present with neurological symptoms, psychiatric disorder or seizures. It is more frequently associated with hypothyroidism or euthyroidism, but it can occur, more rarely, even in hyperthyroidism. The presence of serum thyroid (thyroid peroxidase, thyroglobulin) antibodies is a criterion for the diagnosis of HE. We present the case of an 11-years-old girl with HE with seizures associated with hyperthyroidism with an excellent response to a high dose of steroids.