A trial to investigate teprotumumab subcutaneous administration compared with placebo in male and female adult participants with moderate-to-severe active thyroid eye disease

1. Participant must provide written informed consent. 2. Participant can be male or female and must be between the ages of 18 and 80 years, inclusive, at Screening. 3. Participant must have a clinical diagnosis of Graves’ disease associated with active TED with a CAS ≥ 3 (on the 7-item scale) for the most severely affected eye at Screening and Baseline. 4. Participant must have moderate-to-severe active TED (not sight-threatening but has an appreciable impact on daily life), usually associated with 1 or more of the following: lid retraction ≥ 2 mm, moderate or severe soft tissue involvement and/or inconstant or constant diplopia. (Note: Participants with no diplopia at Baseline will be limited to approximately 25% of the total number enrolled.) 5. Participant has proptosis ≥ 3 mm from Baseline (prior to diagnosis of TED), as estimated by treating physician, and/or proptosis ≥ 3 mm above normal for race and gender. 6. Participant had onset of active TED symptoms (as determined by participant records) within 15 months prior to Baseline. 7. Participants must be euthyroid with the baseline disease under control or have mild hypoor hyperthyroidism (defined as free thyroxine [FT4] and free triiodothyronine [FT3] levels < 50% above or below the normal limits) at Screening. Every effort should be made to correct the mild hypo- or hyperthyroidism promptly and to maintain the euthyroid state for the full duration of the trial. 8. Participant does not require immediate surgical ophthalmological intervention and is not planning corrective surgery/irradiation during the trial. 9. Women of childbearing potential must have a negative serum pregnancy test at Screening and negative urine pregnancy tests at all protocol-specified time points (ie, prior to each dose and throughout participation in the trial). 10. Participant is willing and able to comply with the protocol requirements for the duration of the trial.

1. Participant has decreased best-corrected visual acuity due to optic neuropathy, defined by a decrease in vision of 2 lines on the Snellen chart (or equivalent), new visual field defect or color defect secondary to optic nerve involvement within the last 6 months. 2. Participant has corneal decompensation unresponsive to medical management. 3. Participant has a decrease in CAS of ≥ 2 points between Screening and Baseline. 4. Participant has a decrease in proptosis of ≥ 2 mm between Screening and Baseline. 5. Participant had prior orbital irradiation, orbital decompression or strabismus surgery (excluding childhood strabismus surgeries unrelated to TED/Graves’ disease). 6. Participant is planning to have eyelid surgery during the trial. 7. Participant received periocular botulinum toxin injection within 12 months prior to Screening. 8. Participant has any systemic use of a steroid (IV or oral) or steroid eye drops for the treatment of TED or other conditions within 3 weeks prior to Screening. Exceptions include local administration (excluding periocular), eg, topical, intra-articular, and inhaled steroids, as well as steroids used to treat infusion reactions.