The cause of TED is complex and not fully understood. Almost all patients with Graves' disease have an element of TED, but it does not occur with simple goitres, even though there is over replacement with thyroid hormone. TED can affect people with normally acting thyroid glands (Euthyroid) or even those with under acting thyroid glands (Hypothyroid). TED may precede thyroid problems in 5-25% of cases, however TED typically occurs about 12-18 months after the development of hyperthyroidism (overactivity of the thyroid gland). Smoking is a major factor in the developent of TED.
It is thought TED is an autoimmune disease occurring alongside, but independently of thyroid disease. Antibodies, like those active against the thyroid, may damage the soft tissues in the orbit (the bony socket of the eye) producing inflammation with enlargement of the muscles and the fat around the eye. The progress of TED is unpredictable and is often unrelated to the severity of the thyrotoxicosis (hyperthyroidism). Overall it tends to last 1-3 years with some of the effects being reversible and others, like proptosis, tending to be permanent.