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

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.