Hashimoto's encephalopathy (HE) is a rare neurological syndrome characterized by the presence of positive serum antithyroid antibodies, altered mental status, and clinical response to glucocorticoid therapy. Although HE has been documented in the literature from a medical standpoint, reports on the neuropsychological presentation of this syndrome are scarce. This article presents a literature review of cognitive deficits reported in HE cases. In addition, we describe the case of a 76-year-old Russian speaking woman diagnosed with HE in May 2012. MRIs from her disease course and treatment are presented. Posttreatment neuropsychological testing revealed intact attention and construction and impairment in working memory, processing speed, learning, executive functioning, language, and bilateral fine motor dexterity, all of which impacted her functional abilities. Her profile was inconsistent with the typical dementia process. This case demonstrates the utility of neuropsychological assessment for understanding cognitive and functional consequences of HE. The issue of differential diagnosis with dementia is also discussed.