Thyroid Function Parameters, Thyroid Status, and Vitamin B12 Deficiency in Hospitalized Adults: A Retrospective Cross-Sectional Study in Eastern India

Background: Vitamin B12 deficiency and thyroid dysfunction frequently coexist in hospitalized populations. Prior studies have not stratified patients by thyroid functional status or systematically characterized the relationship between individual thyroid parameters and B12 levels in unselected inpatients. Aim: To evaluate associations between thyroid function parameters (triiodothyronine (T3), thyroxine (T4), thyroid-stimulating hormone (TSH)) and serum vitamin B12; compare B12 deficiency prevalence across five classified thyroid status categories; and identify independent predictors of B12 deficiency in hospitalized adults. Methods: This retrospective cross-sectional study analyzed de-identified biochemical records from 835 unique hospitalized adults (deduplicated from 1,700 initial records) with simultaneous serum vitamin B12, T3, T4, and TSH measurements at a tertiary care hospital in eastern India. Patients were classified into five thyroid categories using established biochemical thresholds. B12 deficiency was defined as <200 pg/mL; a borderline zone (200-299 pg/mL) was additionally analyzed. Spearman's rank correlation, Mann-Whitney U test, Kruskal-Wallis test with Bonferroni correction, and multivariable binary logistic regression were performed. Results: B12 deficiency was identified in 218 patients (26.1%). Deficiency prevalence differed significantly across thyroid categories (Chi-square p = 0.027); subclinical hypothyroid patients had the highest rate (33.5%) and overt hypothyroid patients had the lowest (16.0%). Kruskal-Wallis testing confirmed significant B12 level differences across groups (H = 27.44, p < 0.001), with overt hypothyroid patients having significantly higher median B12 than euthyroid (p_adj = 0.002) and subclinical hypothyroid (p_adj < 0.001) patients. On multivariable logistic regression, increasing age (odds ratio, OR 0.978; 95% CI 0.969-0.987; p < 0.001) and elevated T4 (OR 1.101; 95% CI 1.037-1.168; p = 0.002) were independent predictors. Pseudo R² = 0.050. Conclusion: Vitamin B12 deficiency affects more than one in four hospitalized adults. Subclinical hypothyroidism carries the highest deficiency rate, while overt hypothyroidism paradoxically shows the lowest, the reasons for which require prospective investigation. Serum T4 and age independently predict deficiency. Prospective studies incorporating autoimmune markers and metabolic confirmatory testing are warranted.