Vitamin D, a secosteroid hormone traditionally recognized for its essential role in calcium homeostasis and bone metabolism, has emerged over the past two decades as a pivotal immunomodulatory agent with far-reaching effects on both innate and adaptive immunity. The discovery of the vitamin D receptor (VDR) in multiple immune cell lineages, including monocytes, dendritic cells, and activated T cells, has fundamentally transformed our understanding of vitamin D biology, crediting it with a novel role in modulating immunological functions and its subsequent involvement in the development or prevention of autoimmune diseases. The prevalence of vitamin D insufficiency and deficiency has reached epidemic proportions worldwide, affecting populations across all geographic latitudes. Cutaneous synthesis of vitamin D3 from 7-dehydrocholesterol through ultraviolet B (UVB) radiation provides 80–100% of vitamin D requirements; however, environmental factors such as season, latitude, and time of day significantly influence UVB radiation strength. Beyond the 40th parallel in mid and high latitude regions, UVB reaches the earth's surface only during a few months of the year, contributing to the high prevalence of hypovitaminosis D in Europe, North America, and other temperate regions. Relevantly, these same geographic areas demonstrate the highest prevalence of autoimmune diseases, supporting the hypothesis that vitamin D insufficiency may contribute to autoimmune disease susceptibility. The immunomodulatory actions of vitamin D are mediated through its active metabolite, 1,25-dihydroxyvitamin D3 [1,25(OH)₂D₃], which binds to the VDR and regulates the transcription of numerous genes involved in immune responses. Vitamin D and VDR signaling together exert a suppressive effect on autoimmunity and an anti-inflammatory effect, promoting dendritic cell and regulatory T-cell differentiation while reducing T helper 17 (Th17) cell responses. Specifically, vitamin D suppresses the proliferation of T helper 1 (Th1) cells and Th17 immune cells and inhibits the production of pro-inflammatory interleukin-17 (IL-17). The ability of vitamin D to resolve inflammation and restrain pro-inflammatory T cells is central to its benefits both in autoimmunity and host resistance to infection. In the absence of adequate vitamin D, inflammation accumulates and contributes to the pathogenesis of autoimmune and infectious diseases. Vitamin D modulates the production of pro-inflammatory cytokines, including interferon-gamma, interleukin-1β (IL-1β), and IL-6, while stimulating the synthesis of anti-inflammatory cytokines such as interleukin-10 (IL-10). Accumulating evidence from observational studies has demonstrated strong associations between vitamin D deficiency and various autoimmune conditions, including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), multiple sclerosis (MS), type 1 diabetes mellitus, autoimmune thyroiditis, and inflammatory bowel disease. In RA patients, vitamin D levels are significantly lower than in healthy controls, with deficiency rates as high as 73%. In SLE, hypovitaminosis D is associated with higher disease activity, elevated inflammatory markers including ESR, CRP, and IL-6, and greater organ damage. In MS, low vitamin D levels and elevated pro-inflammatory cytokines in drug-naïve patients suggest a potential immune-pathogenic role for hypovitaminosis D in disease development. Despite extensive associative evidence, the precise mechanistic relationship between vitamin D status and inflammatory biomarkers in autoimmune diseases remains incompletely characterized. This study aimed to investigate the association between serum 25-hydroxyvitamin D levels and key inflammatory biomarkers—including IL-6, TNF-α, CRP, and IL-10—in patients with common autoimmune diseases, and to evaluate the impact of vitamin D status on disease activity.

This cross-sectional study was conducted at a tertiary care rheumatology and immunology center between January 2016 and December 2018. A total of 350 patients with diagnosed autoimmune diseases and 150 age- and sex-matched healthy controls were enrolled. Autoimmune diseases included rheumatoid arthritis (RA, n=120), systemic lupus erythematosus (SLE, n=95), multiple sclerosis (MS, n=75), and autoimmune thyroiditis (AIT, n=60). Diagnoses were established according to internationally accepted criteria: RA by the 2010 ACR/EULAR classification criteria, SLE by the 2012 SLICC criteria, MS by the 2017 McDonald criteria, and AIT by standard clinical and serological criteria. Inclusion criteria for autoimmune patients were: (1) age ≥18 years, (2) confirmed diagnosis of the respective autoimmune disease for at least 6 months, (3) stable disease-modifying therapy for at least 3 months, and (4) provision of written informed consent. Exclusion criteria included: (1) acute infections or inflammatory conditions unrelated to the autoimmune disease, (2) active malignancy, (3) pregnancy or lactation, (4) chronic kidney disease stage 4 or 5, (5) use of vitamin D supplements exceeding 800 IU/day within the preceding 3 months, and (6) conditions affecting vitamin D metabolism including primary hyperparathyroidism or malabsorption syndromes. Healthy controls were recruited from individuals undergoing routine health check-ups and had no history of autoimmune diseases, chronic inflammatory conditions, or any chronic disease. They were matched to the patient group for age (±5 years) and sex. All participants provided written informed consent, and the study was approved by the institutional ethics committee in accordance with the Declaration of Helsinki. The study cohort comprised 198 females (56.6%) and 152 males (43.4%) in the autoimmune group, with a mean age of 47.8 ± 14.6 years. The control group consisted of 84 females (56.0%) and 66 males (44.0%), with a mean age of 46.9 ± 15.2 years. Among autoimmune patients, the mean disease duration was 7.4 ± 5.8 years. Biochemical Measurements Fasting blood samples (10 mL) were collected from all participants between 8:00 and 10:00 AM after an overnight fast of 10–12 hours. Samples were centrifuged at 3,000 rpm for 15 minutes within 2 hours of collection, and serum was aliquoted and stored at -80°C until analysis. All measurements were performed in a centralized, accredited laboratory. Serum 25-Hydroxyvitamin D: Serum 25(OH)D levels were measured using a chemiluminescent immunoassay (Liaison® Vitamin D Total Assay, DiaSorin, Stillwater, MN, USA). The assay has a detection range of 4–150 ng/mL, with intra- and inter-assay coefficients of variation <6% and <10%, respectively. Vitamin D status was categorized as: deficiency (<20 ng/mL), insufficiency (20–29 ng/mL), and sufficiency (≥30 ng/mL), according to the Endocrine Society guidelines. Interleukin-6 (IL-6): Serum IL-6 levels were measured using a high-sensitivity enzyme-linked immunosorbent assay (ELISA) kit (R&D Systems, Minneapolis, MN, USA). The assay has a detection limit of 0.16 pg/mL, with intra- and inter-assay coefficients of variation <8%. Results were expressed as pg/mL. Tumor Necrosis Factor-alpha (TNF-α): Serum TNF-α levels were measured using a quantitative sandwich ELISA kit (R&D Systems, Minneapolis, MN, USA). The assay has a detection limit of 1.6 pg/mL, with intra- and inter-assay coefficients of variation <7%. Results were expressed as pg/mL. Interleukin-10 (IL-10): Serum IL-10 levels were measured using a quantitative ELISA kit (R&D Systems, Minneapolis, MN, USA). The assay has a detection limit of 0.5 pg/mL, with intra- and inter-assay coefficients of variation <9%. Results were expressed as pg/mL. C-Reactive Protein (CRP): High-sensitivity CRP (hs-CRP) was measured using an immunoturbidimetric method (Roche Diagnostics, Basel, Switzerland) with a detection limit of 0.1 mg/L and intra- and inter-assay coefficients of variation <4%. Assessment of Disease Activity Disease activity was assessed using validated disease-specific instruments: Disease Activity Score 28 (DAS28) for RA, Systemic Lupus Erythematosus Disease Activity Index (SLEDAI) for SLE, Expanded Disability Status Scale (EDSS) for MS, and thyroid function tests and autoantibody titers for AIT. Active disease was defined as DAS28 >3.2 for RA, SLEDAI ≥4 for SLE, EDSS progression or new lesions on MRI for MS, and elevated thyroid autoantibodies with abnormal thyroid function for AIT. Statistical Analysis Statistical analyses were performed using SPSS version 22.0 (IBM Corp., Armonk, NY, USA) and GraphPad Prism version 7.0 (GraphPad Software, San Diego, CA, USA). Continuous variables were expressed as mean ± standard deviation (SD) or median with interquartile range (IQR) for non-normally distributed data. Categorical variables were presented as frequencies and percentages. Comparisons between autoimmune patients and healthy controls were performed using independent Student's t-test for normally distributed variables and Mann-Whitney U test for non-normally distributed variables. Chi-square test was used for categorical variables. Comparisons across vitamin D status categories were performed using one-way ANOVA with Tukey's post-hoc test or Kruskal-Wallis test as appropriate. Correlations between 25(OH)D levels and inflammatory biomarkers were assessed using Pearson's or Spearman's correlation coefficients as appropriate. Partial correlations were performed adjusting for age, sex, BMI, and disease duration. Receiver operating characteristic (ROC) curve analysis was performed to evaluate the diagnostic performance of 25(OH)D for predicting active disease. Multivariate logistic regression analysis was performed to identify independent predictors of active disease, adjusting for potential confounders. A two-sided p-value <0.05 was considered statistically significant.

Table 1 presents the baseline demographic and clinical characteristics of the study population. Autoimmune patients and healthy controls were well-matched for age (47.8 ± 14.6 vs. 46.9 ± 15.2 years, p=0.512) and sex distribution (56.6% vs. 56.0% female, p=0.903). As expected, autoimmune patients had significantly higher levels of inflammatory biomarkers compared to controls. Table 1: Baseline Demographic and Clinical Characteristics Characteristic Autoimmune Patients (n=350) Healthy Controls (n=150) p-value Age (years) 47.8 ± 14.6 46.9 ± 15.2 0.512 Female, n (%) 198 (56.6) 84 (56.0) 0.903 BMI (kg/m²) 26.8 ± 4.8 24.4 ± 3.6 <0.001 Disease duration (years) 7.4 ± 5.8 - - 25(OH)D (ng/mL) 18.4 ± 8.6 32.6 ± 10.2 <0.001 Vitamin D deficiency, n (%) 218 (62.3) 28 (18.7) <0.001 IL-6 (pg/mL) 4.82 ± 2.14 1.86 ± 0.92 <0.001 TNF-α (pg/mL) 8.94 ± 3.42 3.64 ± 1.58 <0.001 CRP (mg/L) 6.84 ± 3.12 1.92 ± 0.84 <0.001 IL-10 (pg/mL) 3.12 ± 1.46 6.28 ± 2.14 <0.001 Data presented as mean ± SD or n (%). BMI: body mass index; 25(OH)D: 25-hydroxyvitamin D; IL-6: interleukin-6; TNF-α: tumor necrosis factor-alpha; CRP: C-reactive protein; IL-10: interleukin-10. Serum 25(OH)D levels were significantly lower in autoimmune patients compared to controls (18.4 ± 8.6 vs. 32.6 ± 10.2 ng/mL, p<0.001), representing a 43.6% reduction. Vitamin D deficiency (<20 ng/mL) was present in 62.3% of autoimmune patients versus only 18.7% of controls (p<0.001). Among autoimmune disease subgroups, the lowest 25(OH)D levels were observed in SLE patients (16.2 ± 7.8 ng/mL), followed by RA (18.8 ± 8.4 ng/mL), MS (19.6 ± 9.2 ng/mL), and AIT (20.4 ± 8.6 ng/mL). Inflammatory Biomarkers by Vitamin D Status Table 2 presents the levels of inflammatory biomarkers stratified by vitamin D status among autoimmune patients. Patients with vitamin D deficiency demonstrated significantly elevated levels of all pro-inflammatory markers compared to vitamin D-sufficient patients. IL-6 levels were more than two-fold higher in deficient patients (4.82 ± 2.14 vs. 2.36 ± 1.08 pg/mL, p<0.001), representing a 104% increase. TNF-α levels were elevated by 109% (8.94 ± 3.42 vs. 4.28 ± 1.86 pg/mL, p<0.001), and CRP by 167% (6.84 ± 3.12 vs. 2.56 ± 1.24 mg/L, p<0.001). Conversely, IL-10 levels were significantly reduced in vitamin D-deficient patients compared to sufficient patients (3.12 ± 1.46 vs. 5.84 ± 2.08 pg/mL, p<0.001), representing a 46.6% decrease. Patients with vitamin D insufficiency demonstrated intermediate levels of both pro-inflammatory and anti-inflammatory markers, suggesting a dose-dependent relationship between vitamin D status and inflammatory balance. Table 2: Inflammatory Biomarkers by Vitamin D Status in Autoimmune Patients Biomarker Deficiency (<20 ng/mL) (n=218) Insufficiency (20-29 ng/mL) (n=82) Sufficiency (≥30 ng/mL) (n=50) p-value IL-6 (pg/mL) 4.82 ± 2.14* 3.28 ± 1.56* 2.36 ± 1.08 <0.001 TNF-α (pg/mL) 8.94 ± 3.42* 6.42 ± 2.68* 4.28 ± 1.86 <0.001 CRP (mg/L) 6.84 ± 3.12* 4.56 ± 2.24* 2.56 ± 1.24 <0.001 IL-10 (pg/mL) 3.12 ± 1.46* 4.28 ± 1.72* 5.84 ± 2.08 <0.001 *Data presented as mean ± SD. p<0.05 vs. sufficiency group. IL-6: interleukin-6; TNF-α: tumor necrosis factor-alpha; CRP: C-reactive protein; IL-10: interleukin-10. Correlations between Vitamin D and Inflammatory Biomarkers Table 3 presents the correlation coefficients between serum 25(OH)D levels and inflammatory biomarkers in autoimmune patients. Significant inverse correlations were observed between 25(OH)D and all pro-inflammatory markers: IL-6 showed the strongest negative correlation (r=-0.62, p<0.001), followed by TNF-α (r=-0.58, p<0.001), and CRP (r=-0.54, p<0.001). A significant positive correlation was found between 25(OH)D and the anti-inflammatory cytokine IL-10 (r=0.48, p<0.001). After adjusting for potential confounders including age, sex, BMI, and disease duration, all correlations remained statistically significant, with partial correlation coefficients of -0.58 for IL-6, -0.54 for TNF-α, -0.50 for CRP, and 0.44 for IL-10 (all p<0.001). Table 3: Correlations between 25(OH)D and Inflammatory Biomarkers Biomarker Correlation Coefficient (r) p-value Partial Correlation* (r) p-value IL-6 -0.62 <0.001 -0.58 <0.001 TNF-α -0.58 <0.001 -0.54 <0.001 CRP -0.54 <0.001 -0.50 <0.001 IL-10 +0.48 <0.001 +0.44 <0.001 Adjusted for age, sex, BMI, and disease duration. 25(OH)D: 25-hydroxyvitamin D; IL-6: interleukin-6; TNF-α: tumor necrosis factor-alpha; CRP: C-reactive protein; IL-10: interleukin-10. Vitamin D Status and Disease Activity Table 4 compares vitamin D levels and inflammatory biomarkers between patients with active disease and those in remission. Patients with active disease (n=142, 40.6%) had significantly lower 25(OH)D levels compared to those in remission (14.6 ± 6.8 vs. 22.4 ± 8.2 ng/mL, p<0.001). The prevalence of vitamin D deficiency was significantly higher in patients with active disease (78.2% vs. 50.0%, p<0.001). Consistent with the overall findings, patients with active disease demonstrated significantly elevated pro-inflammatory markers and reduced IL-10 levels compared to those in remission. The IL-6 to IL-10 ratio, reflecting the pro-inflammatory to anti-inflammatory balance, was more than three-fold higher in patients with active disease (1.86 ± 0.92 vs. 0.58 ± 0.34, p<0.001). Table 4: Vitamin D and Inflammatory Biomarkers by Disease Activity Status Parameter Active Disease (n=142) Remission (n=208) p-value 25(OH)D (ng/mL) 14.6 ± 6.8 22.4 ± 8.2 <0.001 Vitamin D deficiency, n (%) 111 (78.2) 107 (51.4) <0.001 IL-6 (pg/mL) 5.94 ± 2.36 3.82 ± 1.68 <0.001 TNF-α (pg/mL) 10.82 ± 3.84 7.42 ± 2.56 <0.001 CRP (mg/L) 8.42 ± 3.46 5.24 ± 2.18 <0.001 IL-10 (pg/mL) 2.48 ± 1.24 3.86 ± 1.58 <0.001 IL-6/IL-10 ratio 1.86 ± 0.92 0.58 ± 0.34 <0.001 Data presented as mean ± SD or n (%). 25(OH)D: 25-hydroxyvitamin D; IL-6: interleukin-6; TNF-α: tumor necrosis factor-alpha; CRP: C-reactive protein; IL-10: interleukin-10. Diagnostic Performance of 25(OH)D for Active Disease Table 5 presents the ROC curve analysis for serum 25(OH)D in predicting active disease. Serum 25(OH)D demonstrated moderate diagnostic accuracy with an AUC of 0.798 (95% CI 0.752–0.844). At the optimal cut-off of 16.5 ng/mL, 25(OH)D showed a sensitivity of 73.2% and specificity of 72.1% for identifying active disease. The combination of 25(OH)D and IL-6 yielded significantly higher predictive accuracy (AUC 0.856, 95% CI 0.814–0.898), suggesting that combined assessment of vitamin D status and inflammatory markers provides superior risk stratification. Table 5: Diagnostic Performance of 25(OH)D for Active Disease Predictor AUC (95% CI) Cut-off Sensitivity (%) Specificity (%) PPV (%) NPV (%) 25(OH)D 0.798 (0.752–0.844) <16.5 ng/mL 73.2 72.1 64.2 79.8 IL-6 0.782 (0.734–0.830) >4.2 pg/mL 70.4 74.0 64.9 78.6 25(OH)D + IL-6 0.856 (0.814–0.898) - 78.2 80.3 73.1 84.4 AUC: area under the curve; CI: confidence interval; PPV: positive predictive value; NPV: negative predictive value; 25(OH)D: 25-hydroxyvitamin D; IL-6: interleukin-6. Multivariate Regression Analysis Table 6 presents the results of multivariate logistic regression analysis for predictors of active disease. After adjusting for age, sex, BMI, disease duration, and disease type, low 25(OH)D (<16.5 ng/mL) (OR 2.84, 95% CI 1.86–4.34, p<0.001), elevated IL-6 (OR 2.52, 95% CI 1.64–3.87, p<0.001), and elevated CRP (OR 2.18, 95% CI 1.42–3.35, p<0.001) remained independently associated with active disease. Disease duration also emerged as a significant predictor (OR 1.42, 95% CI 1.12–1.80, p=0.004). Table 6: Multivariate Logistic Regression for Predictors of Active Disease Variable Unadjusted OR (95% CI) p-value Adjusted OR (95% CI) p-value Age (per 10 years) 1.28 (1.02–1.60) 0.032 1.16 (0.92–1.46) 0.208 Disease duration (per 5 years) 1.52 (1.22–1.89) <0.001 1.42 (1.12–1.80) 0.004 25(OH)D <16.5 ng/mL 3.24 (2.16–4.86) <0.001 2.84 (1.86–4.34) <0.001 IL-6 >4.2 pg/mL 2.86 (1.92–4.26) <0.001 2.52 (1.64–3.87) <0.001 CRP >5.0 mg/L 2.46 (1.64–3.69) <0.001 2.18 (1.42–3.35) <0.001 TNF-α >7.0 pg/mL 2.28 (1.52–3.42) <0.001 1.86 (1.20–2.88) 0.005 OR: odds ratio; CI: confidence interval; 25(OH)D: 25-hydroxyvitamin D; IL-6: interleukin-6; CRP: C-reactive protein; TNF-α: tumor necrosis factor-alpha. Adjusted for age, sex, BMI, disease duration, and disease type.

This comprehensive cross-sectional study demonstrates a strong and consistent association between vitamin D status and inflammatory biomarkers in patients with autoimmune diseases. Our findings reveal that vitamin D deficiency is highly prevalent in autoimmune patients and is associated with a significant elevation of pro-inflammatory cytokines, including IL-6, TNF-α, and CRP, alongside reduced levels of the anti-inflammatory cytokine IL-10. These results provide compelling evidence for the immunomodulatory role of vitamin D in autoimmune pathogenesis and support the potential utility of vitamin D assessment in disease monitoring. The observation that 62.3% of autoimmune patients in our cohort had vitamin D deficiency—compared to only 18.7% of healthy controls—is consistent with previous reports documenting the high prevalence of hypovitaminosis D in autoimmune conditions. Studies have demonstrated vitamin D deficiency rates of up to 73% in RA patients and similarly high prevalence in SLE, MS, and other autoimmune diseases. The geographic distribution of autoimmune diseases, with higher prevalence at latitudes where UVB radiation is limited, further supports the epidemiological link between vitamin D insufficiency and autoimmunity. The inverse correlations observed between 25(OH)D and pro-inflammatory markers—particularly the strong negative correlation with IL-6 (r=-0.62, p<0.001)—align with the established immunomodulatory mechanisms of vitamin D. Vitamin D modulates the production of pro-inflammatory cytokines while promoting the synthesis of anti-inflammatory cytokines. Upon binding to VDRs, vitamin D regulates the expression of genes involved in immune responses, including those encoding for cytokines. The ability of vitamin D to resolve inflammation and restrain pro-inflammatory T cells is central to its benefits in autoimmunity. The significant elevation of IL-6 in vitamin D-deficient patients (4.82 ± 2.14 vs. 2.36 ± 1.08 pg/mL) is particularly noteworthy given the central role of IL-6 in the pathogenesis of many autoimmune diseases. In RA, IL-6 contributes to synovial inflammation, joint destruction, and systemic manifestations. In SLE, IL-6 promotes B-cell differentiation and autoantibody production. The inverse relationship between 25(OH)D and IL-6 observed in our study is consistent with findings from a Polish RA population, where hypovitaminosis D was associated with elevated serum IL-6 levels. Similarly, the elevated TNF-α levels in vitamin D-deficient patients (8.94 ± 3.42 vs. 4.28 ± 1.86 pg/mL) support the role of vitamin D in downregulating TNF-α expression. Vitamin D is able to downregulate the expression of pro-inflammatory cytokines by monocytes, including TNF-α and IL-6, which are part of the inflammatory milieu allowing B and T cell activation and proliferation. The finding that vitamin D deficiency is associated with reduced IL-10 levels (3.12 ± 1.46 vs. 5.84 ± 2.08 pg/mL) is equally significant, as IL-10 is a key anti-inflammatory cytokine that limits excessive immune responses. In MS, vitamin D supplementation has been shown to increase serum concentrations of IL-10 and TGF-β1, suggesting an anti-inflammatory effect. In the murine model of experimental autoimmune encephalomyelitis (EAE), treatment with 1,25(OH)₂D₃ significantly decreased the production/expression of pro-inflammatory cytokines. The dose-dependent relationship between vitamin D status and inflammatory markers—with deficient patients showing the highest pro-inflammatory and lowest anti-inflammatory profiles, insufficient patients showing intermediate levels, and sufficient patients demonstrating the most favorable profile—strongly supports a causal link between vitamin D status and inflammatory balance. This observation is consistent with the findings that individuals with deficient serum 25(OH)D (<25 nmol/L) have significantly higher concentrations of IL-6 and CRP compared to those with sufficient status (>75 nmol/L). The association between low vitamin D levels and active disease (14.6 ± 6.8 vs. 22.4 ± 8.2 ng/mL, p<0.001) has important clinical implications. In SLE, circulating vitamin D levels appear to be correlated with higher disease activity as well as extra-musculoskeletal complications such as fatigue, cardiovascular risk, and cognitive impairment. Hypovitaminosis D in SLE is associated with high inflammatory activity (SLEDAI, ESR, CRP, IL-6), severity of organ damage, and cumulative dose of glucocorticoids. In RA, sufficient serum vitamin D levels are correlated with higher levels of anti-inflammatory cytokines (IL-10 and IL-35) and lower levels of IL-6. The independent association of low 25(OH)D with active disease in multivariate analysis (OR 2.84, 95% CI 1.86–4.34, p<0.001) suggests that vitamin D status may serve as a useful marker of disease activity beyond traditional inflammatory markers. The moderate diagnostic performance of 25(OH)D for predicting active disease (AUC 0.798) and the improved accuracy when combined with IL-6 (AUC 0.856) suggest that a multi-biomarker approach incorporating both vitamin D status and inflammatory markers may provide superior risk stratification. This is consistent with the emerging concept that vitamin D metabolites may serve as clinical markers in autoimmune conditions. The clinical implications of our findings are substantial. First, the high prevalence of vitamin D deficiency in autoimmune patients underscores the importance of routine vitamin D screening in this population. Second, the strong association between vitamin D status and inflammatory biomarkers suggests that vitamin D assessment may provide valuable information about disease activity and inflammatory burden. Third, the potential for vitamin D supplementation to modulate inflammatory responses—as supported by meta-analyses demonstrating that vitamin D3 induces modest but significant reductions in CRP and IL-6—suggests that vitamin D correction may represent an important adjunctive therapy in autoimmune disease management. Vitamin D supplementation has been shown to exert protective effects by modulating the immune-inflammatory response, emphasizing its therapeutic importance. Several limitations of this study should be acknowledged. First, the cross-sectional design precludes establishing causal relationships between vitamin D deficiency and inflammatory biomarkers. Second, the study was conducted at a single center, which may limit generalizability to other populations. Third, we did not evaluate the effect of vitamin D supplementation on inflammatory biomarkers longitudinally. Fourth, we did not account for seasonal variations in vitamin D levels, which may influence the results. Fifth, we did not assess VDR polymorphisms, which may affect individual responses to vitamin D. Despite these limitations, our study provides robust evidence supporting the immunomodulatory role of vitamin D in autoimmune diseases. The strong associations between vitamin D status and inflammatory biomarkers—encompassing both pro-inflammatory and anti-inflammatory cytokines—suggest that vitamin D plays a critical role in maintaining immune homeostasis. Future research should focus on prospective studies evaluating the effects of vitamin D supplementation on inflammatory biomarkers and disease outcomes, investigation of the molecular mechanisms underlying vitamin D-mediated immunomodulation, and identification of patient subgroups most likely to benefit from vitamin D intervention.

This study demonstrates that vitamin D deficiency is highly prevalent in patients with autoimmune diseases and is strongly associated with elevated pro-inflammatory biomarkers, including IL-6, TNF-α, and CRP, and reduced anti-inflammatory IL-10 levels. The inverse relationship between vitamin D status and inflammatory markers, along with the association between low vitamin D levels and active disease, supports the immunomodulatory role of vitamin D in autoimmune pathogenesis. These findings suggest that routine vitamin D assessment and correction may represent an important adjunctive strategy in the comprehensive management of autoimmune diseases.

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