A Study to Learn About The Effects of Felzartamab on Thyroid Function and Its Safety in Adults With Graves' Disease Ages 18 to 75 Years Old

The primary objective of the study is to evaluate the efficacy of felzartamab compared to placebo in participants with Grave's disease (GD). The secondary objectives of the study are to evaluate the safety, efficacy and pharmacokinetics (PK) of felzartamab in participants with GD.

Key Inclusion Criteria:

• Have active disease and are thyroid-stimulating hormone receptor antibodies (TRAb) positive, defined as:

• Free thyroxine (FT4) and free triiodothyronine (FT3), within the reference range,
• suppressed thyroid-stimulating hormone (TSH)
• TRAb levels > upper limit of normal (ULN)
• Participants must be receiving stable dose of anti-thyroid drugs (ATD) for at least 12 weeks prior to randomization.
• Must agree to refrain from blood product donation from Screening through end of study. If a participant terminates early from the study, they must refrain from blood product donation for 90 days following the last dose of study drug.

Key Exclusion Criteria:

• History of thyroidectomy or radioactive iodine therapy.
• ATD dose change within 12 weeks prior to Day1, or anticipated need for dose adjustment prior to randomization (except for protocol-defined safety management).
• Active, moderate-to-severe, or sight threatening thyroid eye disease (TED) (as assessed by the clinical activity score [CAS]), or requirement for imminent or ongoing TED-directed therapy (e.g., systemic glucocorticoids, biologic therapy, orbital radiation, surgery).
• Any history of malignancy within 5 years prior to Screening except for adequately treated in situ carcinoma of the cervix uteri, basal or squamous cell carcinoma or non-melanomatous skin cancer.
• History of hyperthyroidism not caused by GD (e.g. toxic multinodular goiter, autonomous thyroid nodule, acute inflammatory thyroiditis) and/or history or presence of thyroid storm.
• Type 1 diabetes and type 2 diabetes mellitus with hemoglobin A1c (HbA1c) > 8%.
• Known or suspected history of hypersensitivity to felzartamab or its excipients.
• Use of immunosuppressive therapy within 5 half-lives/12 weeks of Screening.
• Co-existing autoimmune diseases that require systemic immunosuppressants (e.g., cyclosporine, methotrexate, biologics, monoclonals).
• Prior use of an anti-Neonatal Fragment Crystallizable Receptor (FcRn) therapy or intravenous immunoglobulin (IVIg) within 6 months of the last dose prior to Screening.
• Any condition that requires chronic use of high-dose steroids.

NOTE: Other protocol-defined Inclusion/Exclusion criteria may apply.