Background: We present a case of a previously well 71-year-old woman who developed rapidly progressive dementia. She had several prolonged hospital admissions and extensive investigations were performed. Her illness was steroid-responsive with clinical features suggestive of Hashimoto’s Encephalopathy (HE). However, multiple thyroid antibody panels acquired were initially normal, becoming elevated during subsequent relapses that occurred 3 years after initial presentation. The case signifies the importance of repeating antithyroid antibody levels in a patient with a clinical picture suggestive of HE. Methods: Case report. Results: The diagnosis of HE was established because of a compatible clinical picture including a relapsing encephalopathy with motor involvement and steroid responsiveness. In addition, although initial antithyroid antibody testing was negative, subsequent attacks were associated with significant elevations and reverted to normal with resolution of the attacks. Conclusions: In a patient with normal levels of antithyroid antibodies and a clinical presentation suggestive of HE, we recommend repeat antithyroid antibody testing to confirm diagnosis. Further studies are necessary to clarify the pathogenic role of elevated antithyroid antibodies in the mechanism of HE.