Graves' disease is caused by a malfunction in the body's disease-fighting immune system, although the exact reason why this happens is still unknown.

One normal immune system response is the production of antibodies designed to target a specific virus, bacterium or other foreign substance. In Graves' disease — for reasons that aren't well understood — the body produces an antibody to one part of the cells in the thyroid gland, a hormone-producing gland in the neck.

Normally, thyroid function is regulated by a hormone released by a tiny gland at the base of the brain (pituitary gland). The antibody associated with Graves' disease — thyrotropin receptor antibody (TRAb) — acts like the regulatory pituitary hormone. That means that TRAb overrides the normal regulation of the thyroid, causing an overproduction of thyroid hormones (hyperthyroidism).

Cause of Graves' ophthalmopathy

This condition results from a buildup of certain carbohydrates in the muscles and tissues behind the eyes — the cause of which also isn't known. It appears that the same antibody that can cause thyroid dysfunction may also have an "attraction" to tissues surrounding the eyes.

Graves' ophthalmopathy often appears at the same time as hyperthyroidism or several months later. But signs and symptoms of ophthalmopathy may appear years before or after the onset of hyperthyroidism. Graves' ophthalmopathy can also occur even if there's no hyperthyroidism.

Although anyone can develop Graves' disease, a number of factors can increase the risk of disease. These risk factors include the following:

• Family history. Because a family history of Graves' disease is a known risk factor, there is likely a gene or genes that can make a person more susceptible to the disorder.
• Gender. Women are much more likely to develop Graves' disease than are men.
• Age. Graves' disease usually develops in people younger than 40.
• Other autoimmune disorders. People with other disorders of the immune system, such as type 1 diabetes or rheumatoid arthritis, have an increased risk.
• Emotional or physical stress. Stressful life events or illness may act as a trigger for the onset of Graves' disease among people who are genetically susceptible.
• Pregnancy. Pregnancy or recent childbirth may increase the risk of the disorder, particularly among women who are genetically susceptible.
• Smoking. Cigarette smoking, which can affect the immune system, increases the risk of Graves' disease. Smokers who have Graves' disease are also at increased risk of developing Graves' ophthalmopathy.

Complications of Graves' disease can include:

• Pregnancy issues. Possible complications of Graves' disease during pregnancy include miscarriage, preterm birth, fetal thyroid dysfunction, poor fetal growth, maternal heart failure and preeclampsia. Preeclampsia is a maternal condition that results in high blood pressure and other serious signs and symptoms.
• Heart disorders. If left untreated, Graves' disease can lead to heart rhythm disorders, changes in the structure and function of the heart muscles, and the inability of the heart to pump enough blood to the body (congestive heart failure).
• Thyroid storm. A rare, but life-threatening complication of Graves' disease is thyroid storm, also known as accelerated hyperthyroidism or thyrotoxic crisis. It's more likely when severe hyperthyroidism is untreated or treated inadequately.

The sudden and drastic increase in thyroid hormones can produce a number of effects, including fever, profuse sweating, vomiting, diarrhea, delirium, severe weakness, seizures, markedly irregular heartbeat, yellow skin and eyes (jaundice), severe low blood pressure, and coma. Thyroid storm requires immediate emergency care.

• Brittle bones. Untreated hyperthyroidism also can lead to weak, brittle bones (osteoporosis). The strength of your bones depends, in part, on the amount of calcium and other minerals they contain. Too much thyroid hormone interferes with your body's ability to incorporate calcium into your bones.