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

This cross-sectional study aims to investigate the association between serum vitamin D levels and endometriosis, and to further explore the expression of vitamin D receptor (VDR) in endometriotic tissues using immunohistochemistry. The study enrolled reproductive-aged women diagnosed with endometriosis who underwent laparoscopic surgery and compared them with matched healthy controls based on age and BMI. Serum levels of 25-hydroxyvitamin D \[25(OH)D\] were measured using a standardized laboratory assay. Endometriotic tissue samples obtained during surgery were analyzed for VDR expression, qualitative-quantified using the H-score method in both stroma and epithelial compartments. Clinical data, including revised American Society for Reproductive Medicine (rASRM) staging, were used to assess disease severity. Additional data were collected via structured interviews covering sun exposure, reproductive history, lifestyle, and hormonal treatment. Tissue staining and scoring were reviewed by two blinded pathologists to ensure reproducibility. Data entry validation was performed by cross-checking case report forms with original clinical records, and double data entry methods were used to minimize errors. Sample size was determined based on prior effect size estimates from published literature, aiming to achieve a power of 80% and significance level of 5%. The study population was sufficient to detect a clinically meaningful difference in vitamin D status between cases and controls. Missing data were managed using complete case analysis and sensitivity checks. For secondary outcomes, subgroup analyses were conducted based on categorized vitamin D levels (normal, insufficiency, deficiency, severe deficiency). Statistical analysis included: * Descriptive statistics for demographic and clinical characteristics. * Conditional logistic regression or non-parametric equivalents for between-group comparisons. * Logistic regression models to estimate the odds ratios for severe endometriosis by vitamin D status. * Correlation analyses between serum vitamin D levels, VDR expression (H-scores), and rASRM scores. All analyses were performed using STATA version 18.5. The findings aim to clarify the potential role of vitamin D and its receptor in endometriosis pathogenesis and to inform future interventional studies and clinical practice.