Encephalopathy associated with autoimmune thyroid disease (EAATD) is a rare condition associated with Hashimoto's thyroiditis or Graves' disease. It is often underdiagnosed because of the various clinical presentations and lack of widely accepted diagnostic criteria. We report a 46-year-old man who presented with an acute depressive episode and was proven to have encephalopathy associated with Hashimoto's thyroiditis. The exclusive psychiatric manifestations of this case called our attention to the fact that an improved awareness of EAATD in patients with a history of thyroid disease can lead to a timely diagnosis and excellent disease prognosis.