Thyroid eye disease (TED) is a disease marked by swelling of the muscles and fatty tissues surrounding the eyeball within the eye socket (orbit).
The swelling is due to inflammation of these tissues. There is limited space inside the orbit so, as the tissues swell, the eyeball is pushed forward. This causes the clear window in the front part of the eye (the cornea) to lose its protection, which is provided by the eyelids. The eyeball cannot move so easily as the muscles that control it work less well. When the disease is very severe, the nerve connecting the eyeball to the brain can be compressed and damaged. This period of swelling is followed by a healing response.