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 turn on the thyroid and cause it to grow and secrete too much thyroid hormone. The abnormal antibody may also stimulate inflammation in the eyes, and less commonly the skin. 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 stored thyroid hormone. The same symptoms can also be caused by taking too much thyroid hormone in tablet form. These last two forms of excess thyroid hormone are only called thyrotoxicosis, since the thyroid is not overactive.
In rare cases, secondary hyperthyroidism is caused by a tumour of the pituitary gland making too much TSH, leading to constant stimulation of the thyroid gland. More rarely, the pituitary gland becomes insensitive to thyroid hormones, no longer knowing when levels are high and that it should “switch off” the production of TSH.
References:
https://www.thyroid.org/hyperthyroidism/
http://www.medbroadcast.com/condition/getcondition/hyperthyroidism
