Additional Study Information: This study involves the use of an investigational drug called elagolix, in women with heavy menstrual bleeding associated with uterine fibroids. Elagolix suppresses the production of ovarian hormones, estrogen and progesterone. In addition to the investigational drug, you may also receive a small amount of estrogen/progestin medication to decrease some of the side effects from the investigational drug, called add-back therapy. The add-back therapy is being studied with the investigational drug to see if it will reduce the occurrence of symptoms associated with low estradiol levels such as hot flashes, and to prevent potential bone loss, which may occur in women who take the investigational drug. Therefore, the use of the add-back therapy is investigational (experimental) for the purposes of this study. This study is being conducted at approximately 175 research centers in the United States and Puerto Rico. Approximately 600 women with heavy menstrual bleeding associated with uterine fibroids will participate in this study. It is important that you are not intending to become pregnant and will take precautions not to become pregnant during this study. If interested, the study staff will ask you some questions from a screening list to assess whether there are any obvious reasons you may not qualify. Additional information: This study will require the collection of sanitary products during different time points during the study. This study requires using an electronic diary for you to document your symptoms associated with uterine fibroids such as pain, pressure, cramping, back pain, bloating, and urinary problems.