A Study of Efgartigimod PH20 SC Given by Prefilled Syringe in Adults With Thyroid Eye Disease

Inclusion Criteria: * The participant is at least 18 years of age * The participant is capable of providing signed informed consent and following with protocol requirements * The investigator determines active, moderate-to-severe thyroid eye disease (TED) associated with autoimmune thyroid conditions (Graves' disease or Hashimoto's thyroiditis) for the most severely affected eye * The participant has first onset of active TED symptoms within 12 months before screening * The participant must have normal thyroid function with the baseline disease under control or have mild hypo or hyperthyroidism at screening. Every effort should be made to correct the mild hypo or hyperthyroidism promptly and to maintain the normal thyroid function for the full duration of the study * The participant agrees to use birth control consistent with local regulations and the people of child-bearing potential must have a negative blood pregnancy test at screening and a negative urine pregnancy test before receiving the study drug Exclusion Criteria: * Optic neuropathy (damage to optic nerve), defined as new visual field defect (blind spot), relative afferent pupillary defect (pupils respond differently to light), or color defect secondary to optic nerve involvement within the 6 months before screening * Corneal decompensation (swelling of the cornea) unresponsive to medical management * Previous orbital irradiation or surgery for TED * Use of some medications before screening (more information is found in the protocol) * Known autoimmune disease or any medical condition that would interfere with an accurate assessment of clinical symptoms of TED or puts the participant at undue risk * History of malignancy, cancer, unless considered cured by adequate treatment with no evidence of recurrence for ≥3 years. Adequately treated participants with the following cancers can be included at any time: Basal cell or squamous cell skin cancer, Carcinoma in situ of the cervix, Carcinoma in situ of the breast, Incidental histological findings of prostate cancer * Clinically significant active infection that is not sufficiently resolved in the investigator's opinion or positive serum test at screening for active infection with any of the following: Hepatitis B virus (HBV), Hepatitis C virus (HCV), HIV * Current participation in another interventional clinical study or previous participation in an efgartigimod clinical study and at least 1 dose of study drug received or has received at least 1 dose of commercially available efgartigimod * Known hypersensitivity to study drug or one of its excipients (inactive ingredients) * History of or current alcohol, drug, or medication abuse within 12 months before screening as assessed by the investigator * Pregnant or lactating state or intention to become pregnant during the study * Live or live-attenuated vaccine received \<4 weeks before screening The complete list of exclusion criteria can be found in the protocol.