Anti-N-methyl-D-aspartate receptor encephalitis is a life-threatening medical emergency that can be clinically misperceived as Hashimoto's encephalopathy. We present a case of anti-N-methyl-D-aspartate receptor encephalitis in an otherwise healthy young female with subclinical hypothyroidism without an associated ovarian teratoma. She was first misdiagnosed as Hashimoto's encephalopathy due to delirium and behavioral changes, seizures, psychosis, and increased amount of thyroid peroxidase and thyroglobulin antibodies in serum. Final diagnosis was established by third week following presentation with the detection of anti-N-methyl-D-aspartate receptor antibodies in her cerebrospinal fluid. After treatment with intravenous immunoglobulin, methylprednisolone, and amisulpride, she recovered significantly with minimal sequelae at 3-week follow-up.