The goal of this cross-sectional prospective matching study is to evaluate whether serum vitamin D levels and vitamin D receptor (VDR) expression are associated with the presence and severity of endometriosis in women of reproductive age.

The main questions it aims to answer are:

• Is there a difference in serum 25-hydroxyvitamin D [25(OH)D] levels between women with endometriosis and healthy controls?
• Is there a correlation between serum vitamin D levels and VDR expression in endometriotic tissues?
• Is there an association between vitamin D status and the severity of endometriosis based on the revised American Society for Reproductive Medicine (rASRM) staging?

Researchers will compare participants with endometriosis to matched healthy controls (1:2 ratio).

Participants will:

• Provide blood samples for serum 25(OH)D measurement.
• Undergo surgical tissue sampling (for the endometriosis group) to assess VDR expression using immunohistochemistry staining.
• Complete structured questionnaires regarding sun exposure, lifestyle, and clinical characteristics.

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.

Inclusion Criteria Endometriosis Group:

• Women diagnosed with ectopic endometriotic tissue:
• Have a scheduled surgery appointment at Sri Nakarin Hospital.
• Are aged 18 years or older.

Inclusion Criteria Control Group:

• Women undergoing examination at the outpatient department of Sri Nakarin Hospital.
• No history of gynecological diseases or previous treatment for gynecological conditions associated with lower abdominal pain, infections, or abdominal masses at present.
• Have undergone a gynecological examination within the past year, or if examined during the current visit, the results were normal.
• Have undergone an ultrasound examination within the past year, or if examined during the current visit, the ultrasound results were normal.

Exclusion Criteria:

• Received vitamin D supplements within the past 3 months prior to volunteering.
• History of chronic infectious diseases.
• Previously treated for autoimmune diseases.
• History of polycystic ovarian.
• History of bone disorders or receiving medications affecting bones.
• Irregular menstrual cycles or absence of menstruation for more than 3 months up to 1 year.

Thai women, reproductive age

Follow the Center for Ethics in Human Research, Khon Kaen University submission.