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

In this study, researchers will learn more about the use of felzartamab in participants with Graves' Disease, also known as GD. GD is an autoimmune disease, which means the body's immune system attacks its own healthy cells. In people with GD, the immune system produces abnormal antibodies, called thyroid-stimulating hormone receptor antibodies (TRAb), that attack the thyroid gland. This causes the thyroid to become too active and produce too much hormone, a condition called hyperthyroidism. Participants with GD are often treated with anti-thyroid drugs, or ATDs, which are medicines that help bring thyroid hormone levels back to normal. Felzartamab is designed to target certain immune cells that produce the abnormal TRAb antibodies. The main goal of the study is to learn whether felzartamab can help bring thyroid hormone levels back to normal and allow participants to stop taking ATDs. Participants will receive either felzartamab or placebo during the study. A placebo looks like the study drug but contains no real medicine. The main question that researchers want to answer is: • How many participants have normal thyroid levels without receiving any ATD medicine at Week 24? Researchers will also learn more about the safety of felzartamab and how the body processes the drug. The study will be done as follows: * Participants will be screened to check if they can join the study. * This is a double-blind study, which means neither the participants, study doctor, or site staff will know if participants are receiving felzartamab or a placebo. * Participants will be placed into 1 of 3 groups. Two groups will receive felzartamab while the other receives placebo. * Participants will receive felzartamab or placebo as intravenous (IV) infusions, which are slow injections into a vein using a needle. The Treatment Period will last 20 weeks. * During the Treatment Period, the study doctor may gradually lower the dose of the ATD if a participant's thyroid levels become normal. * Afterwards, participants will enter a follow-up period which will last 12 weeks. * In total, participants will have 22 study visits. Participants will stay in the study for about 9 months (up to 36 weeks).