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 withprotocol requirements

• The investigator determines active, moderate-to-severe thyroid eye disease (TED)associated with autoimmune thyroid conditions (Graves' disease or Hashimoto'sthyroiditis) for the most severely affected eye

• The participant has first onset of active TED symptoms within 12 months beforescreening

• The participant must have normal thyroid function with the baseline disease undercontrol or have mild hypo or hyperthyroidism at screening. Every effort should bemade to correct the mild hypo or hyperthyroidism promptly and to maintain the normalthyroid function for the full duration of the study

• The participant agrees to use birth control consistent with local regulations andthe people of child-bearing potential must have a negative blood pregnancy test atscreening 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 (blindspot), relative afferent pupillary defect (pupils respond differently to light), orcolor defect secondary to optic nerve involvement within the 6 months beforescreening

• 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 anaccurate assessment of clinical symptoms of TED or puts the participant at unduerisk

• History of malignancy, cancer, unless considered cured by adequate treatment with noevidence of recurrence for ≥3 years. Adequately treated participants with thefollowing cancers can be included at any time: Basal cell or squamous cell skincancer, Carcinoma in situ of the cervix, Carcinoma in situ of the breast, Incidentalhistological findings of prostate cancer

• Clinically significant active infection that is not sufficiently resolved in theinvestigator's opinion or positive serum test at screening for active infection withany of the following: Hepatitis B virus (HBV), Hepatitis C virus (HCV), HIV

• Current participation in another interventional clinical study or previousparticipation in an efgartigimod clinical study and at least 1 dose of study drugreceived 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 beforescreening 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.