Elagolix is an orally active, gonadotropin-releasing hormone receptor antagonist approved for the management of endometriosis-associated pain and heavy menstrual bleeding associated with uterine fibroids. Elagolix population pharmacokinetics and factors affecting elagolix exposure in healthy women and women with endometriosis have been reported previously. The purpose of this study was to extend the population pharmacokinetics model with additional modifications to incorporate data from phase III studies of elagolix with hormonal add-back therapy in women with uterine fibroids. Data from 13 clinical studies (a total of 2168 women) consisting of six phase I studies in healthy premenopausal women, four phase III studies in premenopausal women with endometriosis, and three phase III studies in premenopausal women with uterine fibroids were analyzed using a non-linear mixed-effects modeling approach. Elagolix population pharmacokinetics was best described by a two-compartment model with first-order absorption, lag time in absorption, and first-order elimination. Out of the covariates tested on elagolix apparent clearance, apparent volume of distribution, and/or relative bioavailability, only organic anion transporting polypeptide 1B1 genotype status and body weight had a statistically significant but no clinically meaningful effect on elagolix relative bioavailability and apparent volume of distribution, respectively. There were no clinically meaningful differences in elagolix population pharmacokinetics in healthy women or women with endometriosis or uterine fibroids. Elagolix population pharmacokinetics modeling did not reveal any patient-related factors or clinical parameters that would require dose adjustments for the approved dosage of 300 mg twice daily with estradiol 1 mg /norethindrone acetate 0.5 mg daily, in women with heavy menstrual bleeding associated with uterine fibroids. ClinicalTrials.gov identifiers: NCT01620528 (EM-1), NCT01760954 (EM-1-Extend), NCT01931670 (EM-2), NCT02143713 (EM-2-Extend), NCT02654054 (UF-1), NCT02691494 (UF-2), NCT0295494 (UF-Extend).