Hashimoto's encephalopathy (HE) is a completely treatable but relapsing neuroendocrine disorder associated with Hashimoto's thyroiditis (HT). Though numerous neurological manifestations are often associated with thyroid disorder, this entity is less documented. We are reporting a case of HE in a 65 year old female presenting with sudden onset deep coma following an attack of mild fever and vomiting for two days. Patient was a known diabetic. Hypoglycemic coma, diabetic ketacidosis and hyperosmolar coma were excluded by laboratory investigations. High blood sugar was corrected with insulin. She had hyponatremia and hypokalemia which were corrected with electrolyte replacement. Liver function tests were normal, but serum ammonia was mildly raised. CSF study was normal. Despite correction of her metabolic derangements patient failed to regain her consciousness. CT scan of brain was normal. MRI of brain revealed diffuse brain atrophy. Patient's thyroid function tests were normal but anti-thyroid peroxidase (anti-TPO) antibody was highly raised. EEG showed diffuse slow wave pattern. Intravenous dexamethasone (24 mg/d) was started. Patient regained consciousness slowly over a period of one month. To reduce the toxicity of steroid, oral azathiprine 50 mg/day was added later with tapering of steroid dose. HE must be kept in mind in comatose patients when other metabolic, infective and structural neurological causes have been excluded. Proper and timely treatment can salvage the patient.