Analysis of Vitamin D and VDR Expression in Women with Advanced Endometriosis: A Case–Control Study in Thailand

<b>Background:</b> Vitamin D has anti-inflammatory and immunomodulatory properties that may influence the pathophysiology of endometriosis. This study investigated the association between vitamin D levels and endometriosis, and vitamin D receptor (VDR) expression in endometriotic tissue. <b>Methods:</b> A cross-sectional study was conducted involving 36 patients with endometriosis and 72 healthy control women, matched for age and BMI. Serum 25-hydroxyvitamin D levels were measured and categorized into four statuses (normal, insufficiency, deficiency, and severe deficiency). Endometriotic tissue samples were examined for VDR expression using immunohistochemistry and qualitatively quantified using histo-scores (H-scores). Endometriosis severity was assessed using the revised criteria of the American Society for Reproductive Medicine (rASRM). <b>Results:</b> No statistically significant difference in vitamin D levels between the groups (20.45 vs. 21.10 ng/dL, <i>p</i> = 0.190) was observed, even after adjusting for residence, body sunscreen use, pregnancy, and contraceptive use. VDR expression exhibited significantly higher H-scores in endometriotic epithelial cells than in stromal cells (209.51 vs. 73.32; <i>p</i> < 0.001). Additionally, the VDR H-score in both cell compartments showed no significant difference according to vitamin D status. No statistically significant association was found between vitamin D levels, VDR expression, or disease severity. The odds of severe endometriosis were 2.17 (95% CI: 0.14-33.80) for vitamin D insufficiency and 4.33 (95% CI: 0.24-115.67) for deficiency compared with normal vitamin D. <b>Conclusions:</b> There was no statistically significant association between vitamin D levels and endometriosis and VDR.