Background: Vitamin D is increasingly associated with the pathology of cognition and mental illness. Vitamin D receptors have been detected on neurons that regulate behavior.
Objective: To assess vitamin D serum concentrations in patients with major depression and schizophrenia as compared to healthy controls and to determine if a correlation exists between serum levels of vitamin D and disease activity.  
Methods: We recruited 50 patients with schizophrenia and compared them to 33 patients with major depression and 50 controls with no major psychopathology. The Positive and Negative Syndrome Scale (PANSS) for schizophrenia and the Hamilton Depression scale for depression were administered on the same day the blood samples were drawn. We used LIAISON® 25-OH vitamin D (DiaSorin) immunoassay to measure serum concentrations of 25-OH vitamin D.
Results: Lower serum vitamin D concentrations were detected among patients with schizophrenia (15.0 ± 7.3 ng/ml) compared to patients with depression (19.6 ± 8.3 ng/ml) and to controls (20.2 ± 7.8 ng/ml, P < 0.05). We found no correlation between disease activity, measured by the PANSS score, and vitamin D levels.   
Conclusions: Serum vitamin D levels were lower in patients with schizophrenia as compared to patients with depression and to healthy controls. No correlation was found between serum concentration and disease activity. Additional studies are needed to elucidate the role of vitamin D in the autoimmune mechanism and in the pathogenesis of schizophrenia.