Psychiatric disorders exhibit significant symptomatic and etiopathological heterogeneity, complicating diagnosis and treatment. Hematological parameters may serve as indicators of overall health and predictors of psychiatric symptom manifestation and remission, particularly in long-term hospitalized patients. This study evaluated hematological and biochemical markers, including vitamin B12, vitamin D, and glucose levels, to explore their potential role in psychiatric disorders and disease progression. Methods. This prospective observational study was conducted from 1 January to 31 December 2022, at the M. Kaczyński Neuropsychiatric Hospital in Lublin, following ethical guidelines. The study included 28 psychiatric inpatients (18 women, 10 men) diagnosed with mental and behavioral disorders (ICD-10: F03, unspecified dementia, and F06.2, organic delusional disorder) and 10 controls without psychiatric diagnoses. Blood samples from both groups underwent hematological and biochemical analyses. Statistical tests included the Shapiro–Wilk test, Kruskal–Wallis test, and Tukey’s multiple range test. Results. Psychiatric patients had significantly lower vitamin B12 (278.00 pg/mL vs. 418.50 pg/mL, p = 0.026) and severe vitamin D deficiency (3.00 ng/mL vs. 26.00 ng/mL, p < 0.001). Hematocrit levels were also lower (38.00% vs. 41.30%, p = 0.033), suggesting anemia risk. No significant differences in glucose levels were found. Reduced mean platelet volume and altered leukocyte subtypes suggested immune dysregulation. Conclusions. Nutritional deficiencies, particularly in vitamin B12 and D, play a critical role in psychiatric disorders. Routine screening and targeted supplementation should be integral to psychiatric care. Addressing these deficiencies may improve treatment outcomes, reduce symptom severity, and enhance patient well-being. Integrating metabolic and nutritional assessments into psychiatric practice is essential for advancing research and clinical management.