A Phase 3B Study to Evaluate Bone Mineral Density With Long-Term Use of Relugolix Combination Tablet in Women With Uterine Fibroids or Endometriosis

Key Inclusion Criteria: * Is a premenopausal woman, 18 to 50 years of age (inclusive); * A diagnosis of uterine fibroids confirmed by imaging or review of medical records and reports heavy menstrual bleeding negatively affecting quality of life. or * A diagnosis of endometriosis that is associated with moderate to severe pain.; * If at risk of pregnancy is willing to avoid pregnancy for 4 years (the duration of the treatment period) using nonhormonal methods of contraception. * Has a negative serum pregnancy test at the screening visit and a negative urine pregnancy test at the allocation visit (or Month 12 if entering from MVT-601-050 \[NCT04756037; SERENE\]); * In good physical and mental health based on medical, surgical, and gynecological history as well as physical, gynecological, and breast examinations, clinical laboratory test results, and vital sign measurements; * Has a body mass index ≥ 18 kg/m\^2. Key Exclusion Criteria: * Has a weight or body habitus that exceeds the limit of the DXA scanner or has a condition that precludes an adequate DXA measurement at the lumbar spine or proximal femur * Has a DXA result demonstrating the following criteria at any anatomic site (lumbar spine, total hip, femoral neck): 1. For patients entering de novo a Z-score ≤ -1.5 or T-score ≤ -2.0 (if ≥ 40 years of age) 2. For patients entering from MVT-601-050 (NCT04756037; SERENE) a 12-month on-treatment DXA demonstrating Z-score ≤ -2.0, T-score ≤ -2.5 (if ≥ 40 years of age), or BMD loss ≥ 8% compared with pre-treatment baseline; * Screening 25-OH vitamin D level \< 12 ng/mL (patients with 25-OH vitamin D deficiency with levels ≥ 12 to \< 20 ng/mL are permitted if supplementing with vitamin D or if vitamin D supplementation is started in the screening period); * Has a history of or currently has Cushing's Syndrome, Rheumatoid Arthritis, metabolic bone disease, uncorrected hyperparathyroidism, Paget's disease of the bone, collagen vascular disease, Marfan's syndrome, Ehlers-Danlos syndrome (if confirmed on genetic testing or meets definitive criteria for hypermobility type), chronic kidney disease (CKD) stage 3 or greater with glomerular filtration rate (GFR) \< 60 mL/min/m2 using Modification of Diet in Renal Disease (MDRD) method, hyperprolactinemia, known pituitary adenoma, hyperthyroidism, anorexia nervosa, bulimia (within the last year), abnormal bone mineral metabolism (eg, hypophosphatemia). Patients whose hyperparathyroidism or hyperthyroidism has been successfully treated or whose hyperprolactinemia has been successfully treated are allowed; * History of low trauma (fragility) fracture. * Past history of use or current use of medication used to treat bone loss other than calcium and vitamin D preparations; * Prior use of depot-medroxyprogesterone acetate for a treatment period \> 2 years (if treatment occurred within the past 5 years) or prior use of GnRH agonist or antagonist for \> 12 months total (unless directly entering from MVT-601-050 \[NCT04756037; SERENE\]); * Malabsorptive disease (including, but not limited to, inflammatory bowel disease and gastric bypass surgery); * Current breast cancer, history of breast cancer or other hormone-sensitive malignancy, at increased risk for hormone-sensitive malignancy, or taking an aromatase inhibitor for breast cancer treatment or prevention * History of organ transplantation or history of bone marrow * BIRADS ≥ 3 Mammogram at entry (or within the past 6 months). * Has a known human immunodeficiency virus (HIV) infection or at high risk of contracting HIV * Has a current psychiatric disorder that would, in the investigator or medical monitor's opinion, impair the ability of the patient to participate in the study or would impair interpretation of their data. * Is currently using a hormonal intrauterine device or contraceptive implant, hormonal contraceptive, or other prohibited medication and is unwilling to discontinue this hormonal contraception