Hashimoto's encephalopathy (HE) is a rare disorder associated with autoimmune thyroiditis. Etiology of HE is not completely understood. High levels of serum antithyroid antibodies are seen in HE. Presentation with otoimmune thyroiditis, cognitive impairment, psychiatric and neurologic symptoms and absence of bacterial or viral enfections are characteristics of HE. HE is a steroid responsive encephalopathy. 60 years old male patient admitted to hospital with forget fulness continuing for 9 months and speech loss starting 2 days ago. Strong positivity of antithyroid antibodies increases the odds for HE. Thyroid function tests showed severe hypothyroidism. Electroencephalography and magnetic resonance imaging results were compatible with HE. HE is diagnosed with differantial diagnosis and exclusion of other reasons. This uncommon disorder is not recognised enough. High titres of serum antithyroid antiboides are always needed for diagnosis. Correct diagnosis requires awareness of wide range of cognitive and clinical presentations of HE.