A Study to Assess Efficacy and Safety of Efgartigimod PH20 SC PFS in Adult Participants With Graves' Disease

The main purpose of this study is to look at how efgartigimod affects thyroid function in adults with Graves' Disease (GD). The study will also check whether efgartigimod is safe and well tolerated. It will look at how efgartigimod is distributed and eliminated in the body, how it changes antibody levels, and how the immune system responds to it. The study consists of a part A double-blinded treatment period, a part B treatment/observation period and a part C open-label treatment/observation period. During the part A and part B treatment periods, participants will receive efgartigimod PH20 SC via Prefilled Syringe (PFS) or placebo. During the part C open-label treatment period, participants will receive efgartigimod PH20 SC PFS. Participation in the different parts of the study will depend on the participant's response to treatment. The total study duration for participants ranges from 63 to 135 weeks, depending on the response to treatment. More information can be found here: https://clinicaltrials.argenx.com/vitalithy

Inclusion Criteria: * Is at least 18 years of age and the local legal age of consent for clinical studies when signing the ICF. * Has a documented diagnosis of GD with TRAb (anti-thyrotropin receptor antibody) levels \>=ULN (upper limit of normal) at screening. * Has active hyperthyroidism due to GD with TSH (thyroid-stimulating hormone) \