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BACKGROUND Steroid-responsive encephalopathy associated with autoimmune thyroiditis (SREAT) can manifest with a wide range of neurological and psychiatric symptoms. CASE PRESENTATION A previously healthy man in his late twenties was admitted several times over the course of half a year. He had acute episodes of reduced consciousness, involuntary movements and psychotic symptoms (e.g. hallucinations and delusions). Initial examinations were normal except for a positive urine drug screen (tetrahydrocannabinol), and the patient was diagnosed with cannabinoid intoxication. During the next admission cerebrospinal fluid analysis showed mild pleocytosis. Screening for anti-neuronal antibodies was negative, but anti-thyroid peroxidase antibodies were detected in serum and cerebrospinal fluid. He was successfully given steroid treatment on a tentative diagnosis of SREAT, but relapsed when the steroids were discontinued. After receiving a prolonged steroid treatment with gradual dose reduction over a year, he remains symptom-free 18 months after treatment discontinuation. INTERPRETATION The diagnostic delay might have been mitigated with an earlier inclusion of neuroimmunological disorders in the differential diagnosis. Unexplained pleocytosis in the cerebrospinal fluid in the presence of paroxysmal neuropsychiatric symptoms should trigger an investigation that includes autoimmune encephalopathies.