TEPEZZA® (Teprotumumab-trbw) Post-Marketing Requirement Study

Participants will be screened for eligibility within 4 weeks prior to the Baseline (Day 1) Visit. Approximately 300 participants who meet the trial eligibility criteria will be randomized on Day 1 in a 1:1:1 ratio in a double-masked fashion (stratified by CAS(clinical activity score) \[≥3 (active) or \<3 (inactive)\] and disease severity \[severe disease, defined as both proptosis above normal for race and gender with binocular diplopia at Baseline vs. non-severe disease\]) to receive: * 4 infusions of teprotumumab (10 mg/kg for the first infusion and 20 mg/kg for the remaining 3 infusions) (Cohort 1) followed by 4 infusions of: * Placebo if a participants is a treatment responder at Week 12 or * Teprotumumab 20 mg/kg if a participant is a treatment non-responder at Week 12 * 8 infusions of teprotumumab (10 mg/kg for the first infusion and 20 mg/kg for the remaining 7 infusions) (Cohort 2) * 16 infusions of teprotumumab (10 mg/kg for the first infusion and 20 mg/kg for the remaining 15 infusions) (Cohort 3) Three weeks following the final infusion of the Initial Treatment Period, there will be a comprehensive End-of-Initial Treatment Visit at Week 24 (Cohorts 1 and 2)/Week 48 (Cohort 3). At this visit, all participants will be assessed for treatment response. Proptosis responders in all cohorts and non-responders in Cohorts 1 and 2 who choose not to receive a second treatment course, will enter a 52 week Initial Follow-up Period. Proptosis non-responders in Cohorts 1 and 2 who choose to receive a second treatment course (8 infusions) of teprotumumab will receive an infusion q3W. Proptosis non-responders in Cohort 3 are not eligible for a second treatment course following initial treatment. Participants in any of the 3 cohorts who are proptosis responders following the Initial Treatment Period and who flare during the Initial Follow up Period will be eligible to receive re treatment. Acquired from Horizon in 2024