This is a study to assess the efficacy, safety, and tolerability of IMVT-1402 in adult participants with Graves' disease (GD) who are hyperthyroid despite antithyroid drug (ATD) treatment.

The primary objective of this study is to evaluate the efficacy of IMVT-1402 versus placebo as assessed by T3 (total triiodothyronine [T3] or free triiodothyronine [FT3]), free thyroxine (FT4), thyroid-stimulating hormone (TSH), and ATD dose at Week 26.

Inclusion Criteria:

• Participants with the ability to understand the requirements of the trial, provide written informed consent, and comply with the trial protocol procedures.
• Male or female participants aged ≥ 18 years.
• Participants with diagnosis of GD who are hyperthyroid despite ATD treatment.
• Other, more specific inclusion criteria are defined in the protocol.

Exclusion Criteria:

• Have previously been successfully treated with radioactive iodine (RAI) therapy or have undergone total thyroidectomy.
• Have an autoimmune disease other than GD requiring treatment that, in the Investigator's judgment, puts the participant at undue risk.
• Have moderate-to-severe active thyroid eye disease (TED) and are expected to require immediate surgical intervention and/or are planning corrective surgery/irradiation or medical therapy for TED during study participation.
• Additional exclusion criteria are defined in the protocol.