The aim of this randomised clinical trial study is to evaluate whether supplementation with vitamin D could reduce the risk of uterine fibroids in reproductive stage women.

A total of 1160 hypovitaminosis D patients (12 ng/ml ≤ serum 25-hydroxyvitamin D3 < 20 ng/ml) without uterine fibroids will be randomly assigned in a ratio of 1:1 to two groups: intervention group and control group. The intervention group will receive 1600 IU/d of vitamin D3; the control group will receive regular follow-up. Patients will be followed up at the same time points for 24 months. The outcome measure is the incidence of uterine fibroids in different groups.

Inclusion Criteria:

1. Volunteer to participate in the study with informed consent;
2. Females aged 35-50 who are confirmed with a normal, fibroid-free uterine structure, by means of transvaginal or abdominal ultrasonography;
3. Serum25-hydroxyvitamin D3 <20 ng/ml, ≥ 12ng/ml.

Exclusion Criteria:

1. Use of sexual hormone, mifepristone, gonadotropin-releasing hormone agonist (GnRHa), or other medication which is likely to interfere with uterine fibroids within 3 months;
2. Pregnancy, lactation, postmenopause, or planned pregnancy within two years;
3. Allergic to vitamin D3;
4. Suspected or identified as other tumors of genital tract;
5. History of hysterectomy or myomectomy;
6. History of osteoporosis or vitamin D deficiency taking vitamin D supplements within previous one month;
7. History of hyperparathyroidism, infectious diseases (tuberculosis, AIDS), autoimmune diseases, or digestive system diseases (malabsorption, crohn disease and dysentery);
8. Alanine aminotransferase (ALT) or aspartate transaminase (AST) more than 3 times of the normal upper limit, total bilirubin (TBIL) more than 2 times of the normal upper limit;
9. Creatinine levels ≥ 1.4 mg/dL (123μmol/L) or creatinine clearance ≤ 50 ml/min;
10. History of malignant tumors;
11. Simultaneous participation in another clinical study with investigational medicinal product(s) or researcher thinks the subjects are not suitable for this trial.