Treatment of Graves' Orbitopathy to Reduce Proptosis with Teprotumumab Infusions in an Open-Label Clinical Extension StudyOPTIC-X

- Written informed consent. - Completed the 24-week double-masked Treatment Period in Study HZNP-TEP-301 (NCT03298867). - Proptosis non-responder (< 2 mm reduction in proptosis in the study eye) at Week 24 of Study HZNP-TEP-301 OR proptosis responder at Week 24 who relapses during the Follow-Up Period of Study HZNP-TEP-301. - Participant must be euthyroid with the baseline disease under control, or have mild hypo- or hyperthyroidism (defined as free thyroxine [FT4] and free triiodothyronine [FT3] levels < 50% above or below the normal limits) at the most recent clinic visit. 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 clinical trial. - Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) ≤3 times the upper limit of normal (ULN) or serum creatinine <1.5 times the upper limit of normal (ULN; according to age) at the most recent clinic visit. - Diabetic participants must have well-controlled disease (defined as hemoglobin A1C [HbA1c] < 9.0% at most recent clinic visit). - Does not require immediate surgical ophthalmological intervention and is not planning corrective surgery/irradiation during the course of the study. - Women of childbearing potential must have a negative urine pregnancy test at Baseline/Day 1. Participants who are sexually active with a non-vasectomized male partner must agree to use 2 reliable forms of contraception during the trial and continue for 180 days after the last dose of study drug. One of the 2 forms of contraception is recommended to be hormonal, such as an oral contraceptive. Hormonal contraception must be in use for at least one full cycle prior to Baseline. Highly effective contraceptive methods (with a failure rate less than 1% per year), when used consistently and correctly, includes implants, injectables, combined oral contraceptives, some intrauterine devices (IUDs), sexual abstinence or vasectomised partner. - Male participants must be surgically sterile or, if sexually active with a female partner of childbearing potential, must agree to use a barrier contraceptive method from Baseline through 180 days after the last dose of study drug. - Participant is willing and able to comply with the prescribed treatment protocol and evaluations for the duration of the study. - Has not received any treatment for TED since Week 24 of the of the HZNP-TEP-301 study.

- Participants will be ineligible if, in the opinion of the Investigator, they are unlikely to comply with the study protocol or have a concomitant disease or condition that could interfere with the conduct of the study or potentially put the participant at unacceptable risk. - The exclusion criteria (except those related to screening) of protocol HZNP-TEP-301(NCT03298867) also apply to this open-label extension study.