Case Presentation: This is the case of a 51-year-old male without prior medical or psychiatric history who developed altered sensorium, cognitive dysfunction, mutism and disorganized behavior of 3 weeks of evolution causing severe impairment over his ability to self-care. He was transferred to our institution from a psychiatric hospital since he underwent 2 weeks of treatment without improvement of his symptoms. He had no headaches, fever, weight loss, recent travels or viral syndrome, seizures, blurry vision, focal neurological deficits or history of substance abuse. Physical examination was remarkable for an awake, alert and hypervigilant patient not oriented to person, time or place, who had a “blank” and intimidating stare. He did not follow simple commands and gave delayed and inappropriate responses to simple questions. He had no focal neurological deficits, dystonia, tremors, fasciculation, meningismus, cranial nerves palsies, however, diffuse hyperreflexia was noted.

Discussion: CBC, urinalysis, toxico logy, liver and renal function tests were unremarkable. Head CT scan without IV contrast was negative. Lumbar puncture showed normal opening pressure, corrected WBC count of 4 (neutrophilic predominance), high protein levels (66) and normal glucose levels. These findings were highly suspicious for meningoencephalitis therefore broad-spectrum intravenous antibiotics as well as acyclovir were started. EEG was remarkable for diffuse slow background activity. Other CSF studies such as culture, HSV-1 PCR, cryptococcal antigen, VDRL, Men Panel and anti-NMDAR antibodies were negative. In view of these negative tests, autoimmune vs paraneoplastic encephalitis were highly suspected. Whole-body CT scan revealed no evidence of neoplasm and rheumatologic tests were negative. Thyroid function tests revealed euthyroid state although anti-TPO antibodies were detected. Brain MRI showed bilateral supratentorial periventricular white matter hyperintensities (T2/FLAIR) that are typical of Hashimoto en cephalitis in the context of subacute encephalopathy with positive anti-TPO serology, abnormal EEG and lumbar puncture. High dose steroids were started, which after a week resulted in clinical improvement and increased functionality.

Conclusions: Hashimoto encephalitis (HE) is a rare disease characterized by the acute or subacute development of cognitive dysfunction and altered level of consciousness often associated with seizures, myoclonus or psychosis in the setting of positive anti-thyroperoxidase (TPO) antibodies. This diagnosis is highly controversial since it is a rare condition without a single test to confirm it and it is often misdiagnosed as a psychiatric illness. For these reasons, the diagnosis of HE continues to represents a major diagnostic challenge for clinicians. Our case serves the purpose of raising awareness of this rare and controversial diagnosis of Hashimoto encephalitis as well as the systematic approach we implemented in order to arrive to this diagnosis. Early recognition is highly imperative since untreated long-standing disease may lead to residual cognitive impairment in over 25% of patients and improper psychiatric management of a medical condition.