We report the case of a 63-year-old woman with thiamine deficiency who showed auditory hallucinations, a delusion of persecution, catatonic stupor, and catalepsy but no neurological symptoms including oculomotor or gait disturbance. Brain MRI did not show high-intensity T2 signals in regions including the thalami, mamillary bodies, or periaqueductal area. Her thiamine concentration was 19 ng/mL, only slightly less than the reference range of 20-50 ng/mL. Her psychosis was unresponsive to antipsychotics or electroconvulsive therapy, but was ameliorated by repetitive intravenous thiamine administrations at 100-200 mg per day. However, one month after completing intravenous treatment, her psychosis recurred, even though she was given 150 mg of thiamine per day orally and her blood concentration of thiamine was maintained at far higher than the reference range. Again, intravenous thiamine administration was necessary to ameliorate her symptoms. The present patient indicates that the possibility of thiamine deficiency should be considered in cases of psychosis without neurological disturbance and high-intensity T2 MRI lesions. Also, this case suggests that a high blood thiamine concentration does not necessarily correspond to sufficient thiamine levels in the brain. Based on this, we must reconsider the importance of a high dose of thiamine administration as a therapy for thiamine deficiency. The validity of the reference range of the thiamine concentration, 20-50 ng/mL, is critically reviewed.