Hashimoto's encephalopathy (HE) is a brain disease associated with autoimmune thyroid disease. Over 100 articles have been published on the topic, especially in connection with hypothyroidism. In addition to an overview of the relevant literature, we also report a case of a female patient with a rare association of HE with thyreotoxicosis. The patient presented with varied clinical-neurological symptoms: she had convulsions, psychotic symptoms, altered consciousness Examinations (laboratory tests, CT, MRI, EEG, CSL exams) detected increased level of proteins in the liquor, episodes of rhythmic delta activity on EEG, increased antithyreoidal antibody titre (TOOab, TGAb, TRAb) in serum. After initial treatment with carbimazole and hydrocortizone, the patient's condition dramatically improved and she was disconnected from artificial lung ventilation, conscious and convulsion-free. During the following 30 days, the patient would get worse after attempts to withdraw glucocorticoids. In spite of thyreotoxicosis, we classified the condition as HE and the patient was fully stabilised after pulse treatment with methylprednisolone. Clinically, the patient was subject to further outpatient follow up, without symptoms of encephalopathy; glucocorticoids were gradually withdrawn and were discontinued completely after another four months. The authors conclude that HE, even though rare, may pass unnoticed due to its symptoms which are similar to many other and more frequent diseases. HE should be considered in patients with potential or known autoimmune thyroidism and atypical neuropsychiatric manifestation responding to corticoid treatment.