This trial will evaluate the effects of Lu AG22515 in adult men and women with moderate-to-severe thyroid eye disease (TED). TED is an autoimmune condition closely related to Graves' Disease.

In people with TED, the healthy tissue behind and around the eye becomes inflamed and swollen. One of the key symptoms of TED is proptosis (bulging eyes). The main goal of this trial is to learn whether treatment with Lu AG22515 improves proptosis in participants with moderate-to-severe TED.

Inclusion Criteria:

• The participant has Graves' disease associated Thyroid Eye Disease (TED) symptoms characterized by:
• ophthalmologic symptom onset <12 months prior to the Baseline Visit
• proptosis ≥3 millimeter (mm) above normal for race and sex measured using the Hertel exophthalmometer (at the Screening Visit and the Baseline Visit) in the most severe eye
• Clinical Activity Score (CAS) ≥3 in the most severe eye at the Screening Visit.
• The participants must be euthyroid or have mild hypo- or hyperthyroidism (defined as free thyroxin [FT4] and/or free triiodothyronine [FT3] levels not exceeding the normal limits +/-50%) at the Screening Visit.

Exclusion Criteria:

• The participant has a decreased best-corrected visual acuity due to optic neuropathy, defined as a decrease in vision of 2 lines on the Snellen chart, new visual field defect, or colour defect secondary to optic nerve involvement, within 6 months prior to the Screening Visit.
• The participant has corneal decompensation unresponsive to medical management.
• The participant has a decrease in proptosis of ≥2 mm between the Screening Visit and the Baseline Visit.
• The participant has a decrease in CAS of ≥2 points between the Screening Visit and the Baseline Visit.
• The participant has had previous orbital irradiation or surgery for TED.
• The participant requires immediate surgical ophthalmological intervention or has other eye conditions impacting the assessments.
• The participant has contraindications for an magnetic resonance imaging (MRI) scan.
• The participant has an active infection at Baseline or recent serious infection (for example, requiring hospitalization) within 8 weeks prior to the Baseline Visit.
• The participant takes any of the following disallowed or restricted concomitant medications (the list is not comprehensive):
• Disallowed: any investigational products within 30 days or 5 half-lives, whichever is longer, prior to the Screening Visit, systemic corticosteroids within 6 weeks prior to the Screening Visit (topical steroids for dermatological conditions, inhaled or intranasal steroids are allowed), systemic immunosuppressive/immunomodulating drugs (for example, rituximab, tocilizumab, methotrexate, cyclosporine, azathioprine, mycophenolate-sodium/mofetil, Janus kinase inhibitors) within 5 half-lives prior to the Screening Visit, live attenuated vaccines within 30 days prior to the Baseline Visit, biotin within 1 day prior to the Screening Visit.
• Allowed with restriction: stable dose for >3 weeks prior to the Screening Visit of selenium. Inactivated/killed/RNA-based vaccinations are allowed provided they are not administered within 5 days before/after any investigational medicinal product (IMP) trial visits.