Disease-Specific Clinical Correlates of Vitamin D in Systemic Lupus Erythematosus and Behçet’s Disease

Background and Objectives: Vitamin D deficiency is common in systemic lupus erythematosus (SLE) and Behçet’s disease (BD). However, whether it is associated with the same clinical parameters in both conditions remains unknown. Our objective is to compare the clinical correlates of serum 25-hydroxyvitamin D [25(OH)D] levels in patients with SLE and BD. Materials and Methods: This exploratory cross-sectional study included 38 patients with SLE and 32 with BD. Spearman’s correlation analyses, partial correlations controlling for sex, and tertile analyses were performed separately for each disease group. Results: Vitamin D deficiency [25(OH)D < 20 ng/mL] was similarly prevalent in SLE (64.9%) and BD (62.5%). However, distinct disease-specific correlation profiles were noted. In SLE, serum 25(OH)D levels were inversely correlated with body mass index (BMI) (r = −0.47, p = 0.003) and parathyroid hormone (PTH) (r = −0.40, p = 0.01), with no significant associations with inflammatory or haematological parameters. In patients with BD, 25(OH)D levels were positively correlated with haemoglobin (r = 0.63, p < 0.001), haematocrit (r = 0.58, p < 0.001), ferritin (r = 0.47, p = 0.008), uric acid (r = 0.48, p = 0.005), transferrin saturation (r = 0.45, p = 0.02), and triglycerides (r = 0.44, p = 0.01), while they were inversely correlated with total iron-binding capacity (r = −0.50, p = 0.01) and erythrocyte sedimentation rate (ESR) (r = −0.44, p = 0.01). After adjusting for sex, the association between 25(OH)D and haemoglobin remained significant (partial r = 0.43, p = 0.01), as did the associations with haematocrit (partial r = 0.42, p = 0.02) and triglycerides (partial r = 0.43, p = 0.02). In contrast, the correlations with ferritin, ESR, uric acid, and transferrin saturation were attenuated and no longer statistically significant. Tertile analyses demonstrated dose–response relationships between vitamin D levels and BMI (p = 0.01) and PTH (p = 0.008) in SLE, and between vitamin D levels and haemoglobin (p = 0.001) and ferritin (p = 0.02) in BD. Conclusions: Despite the similar prevalence of vitamin D deficiency, patients with SLE and BD exhibit distinct vitamin D-associated clinical profiles. Vitamin D status was primarily associated with metabolic parameters in SLE and haematological parameters in BD. These disease-specific patterns may reflect differences in the underlying inflammatory pathways and pathophysiological mechanisms involved.