A trial to investigate the efficacy, safety and tolerability of an intravenous infusion of HZN-001 compared with placebo in treating male and female Japanese participants at least 18 years of age with chronic (inactive) thyroid eye disease

INTERVENTION: Approximately 66 participants who meet the trial eligibility criteria will be randomized on Day 1 in a 2:1 ratio (stratified by tobacco use status and presence of diplopia at Baseline) to receive 8 infusions of HZN‐001 (10 mg/kg for the first infusion and 20 mg/kg for the remaining 7 infusions) or placebo once every 3 weeks (q3W). All participants will enter a 24‐week Double‐masked Treatment Period, during which IP will be infused on Day 1 (Baseline) and Weeks 3, 6, 9, 12, 15, 18 and 21 (with a final visit at Week 24 of the 24‐week Treatment Period). CONDITION: Chronic (Inactive) Thyroid Eye Disease PRIMARY OUTCOME: Proptosis responder rate (defined as the percentage of participants with a >= 2‐mm reduction from Baseline in proptosis in the study eye without deterioration of proptosis [increase >= 2 mm] in the fellow eye) at Week 24 INCLUSION CRITERIA: 1. Participant has provided written informed consent. 2. Participant is male or female at least 18 years old at Screening. 3. Participant has an initial diagnosis of TED >= 2 years but < 10 years prior to Screening. Clinical diagnosis of stable cTED, as determined by participant medical records indicating a CAS <= 2 in both eyes for at least 1 year prior to Screening, or all of the following: a. No progression in proptosis for at least 1 year prior to Screening b. If participant has history of diplopia due to TED, no progression in diplopia for at least 1 year prior to Screening c. No new inflammatory TED symptoms for at least 1 year prior to Screening. 4. Participant has CAS <= 2 at the Screening and Baseline Visits. 5. Participant has proptosis >=3‐mm increase from the participant's baseline (prior to diagnosis of TED), as estimated by treating physician, and/or proptosis >= 18 mm. 6. Participants must be euthyroid with the participant's ba