Thyroid eye disease (TED; also known as Graves’ orbitopathy), causes swollen extraocular muscles and orbital fat. Mechanistically, TED involves lid retraction, oedema and redness of the eyelids and conjunctiva, proptosis, diplopia, and optic neuropathy. Investigation of TED involves assessment of disease activity (inflammation) and disease severity. TED is predominantly mild in 77% of cases, moderate-to-severe in 22%, and rarely sight-threatening in 1% of patients. While most patients with TED have Graves’ hyperthyroidism, up to 5% are euthyroid or even hypothyroid.