This work implements a Phase II, dose-ranging, randomized clinical trial (“The Investigation of Vitamin D and Menstrual Cycles Trial: A Phase II Randomized Trial”) investigating vitamin D’s influence on the hypothalamic-pituitary-ovarian axis. The study will carefully evaluate the hormonal, ovulatory, and overall menstrual cycle changes that occur with vitamin D treatment. Vitamin D supplementation may be a low-cost intervention that improves menstrual cycle function and fertility.