Hashimoto's encephalopathy is an autoimmune inflammatory disease of the brain associated with the production of antithyoid antibodies. The case described in this article illustrates the diversity of clinical presentations of the disease that mimic symptoms of some neurological and mental disorders, difficulties of diagnosis and, at the same time, the importance of the timely diagnosis of this severe but curable disease. Diagnostic criteria of encephalopathy allowing the avoidance of both hypo- and hyper diagnosis of this disease are considered. The authors present the current tactics of its treatment based on the use of corticosteroids and other immunotropic drugs.