What causes hyperthyroidism?

The most common cause (in more than 70% of people) is
overproduction of thyroid hormone by the entire thyroid gland. This
condition is also known as Graves’ disease (see the Graves’ Disease
brochure for details). Graves’ disease is caused by antibodies in the
blood that stimulate the thyroid to grow and secrete too much thyroid
hormone. This type of hyperthyroidism tends to run in families, and it
occurs more often in young women. Little is known about why specific
individuals get this disease. Another type of hyperthyroidism is
characterized by one or more nodules or lumps in the thyroid that may
gradually grow and increase their activity so that the total output of
thyroid hormone into the blood is greater than normal. This condition
is known as toxic nodular or multinodular goiter. Also, people may
temporarily have symptoms of hyperthyroidism if they have a condition
called thyroiditis. This condition is caused by a problem with the
immune system or a viral infection that causes the gland to leak thyroid
hormone. It can also be caused by taking too much thyroid hormone
in tablet form