Overactive thyroid (hyperthyroidism) occurs when your thyroid gland produces too much of the thyroid hormones thyroxine or triiodothyronine.
Overproduction of thyroid hormones can be caused by a number of conditions, which are outlined below.
Graves' disease is the most common cause of overactive thyroid. It can run in families and can occur at any age, although it is most common in women aged 20-40 years old. You are more likely to develop Graves' disease if you smoke.
Graves' disease is an autoimmune condition. This means the immune system mistakes something in the body for a toxic substance and attacks it.
In Graves' disease, it attacks the thyroid gland which leads to an overproduction of the thyroid hormones.
It is not known what triggers the immune system to do this. Like many autoimmune conditions it is thought a combination of both genetic and environmental factors could be involved.
If you have Graves' disease, your eyes may also be affected, causing discomfort and double vision. This is known as Graves' ophthalmopathy. You may find your eyes bulge out, or appear more prominent.
See complications of an overactive thyroid gland for more information on Graves' ophthalmopathy.
It is possible for lumps to develop in your thyroid gland. These are known as nodules. It is not known why nodules develop, but they are usually non-cancerous (benign).
However, the nodules can contain abnormal thyroid tissue, which affects the normal production of thyroxine or triiodothyronine, causing overactive thyroid. Nodules that contain abnormal thyroid tissue are described as toxic.
Toxic thyroid nodules account for about 1 in 20 cases of hyperthyroidism.
Iodine contained in the food you eat is used by your thyroid gland to produce the thyroid hormones thyroxine and triiodothyronine. However, taking additional iodine in supplements can cause your thyroid gland to produce too much thyroxine or triiodothyronine.
This is known as iodine-induced hyperthyroidism, sometimes referred to as Jod-Basedow phenomenon. It usually only occurs if you already have nodules in your thyroid gland.
Amiodarone is a type of medication known as an anti-arrhythmic, which helps to control an irregular heartbeat (atrial fibrillation). If you have non-toxic nodules in your thyroid gland, taking amiodarone can cause hyperthyroidism because it contains iodine.
Amiodarone can cause a type of hyperthyroidism usually more severe and difficult to treat through a harmful effect on thyroid tissue. This type of hyperthyroidism is called amiodarone-induced hyperthyroidism.
Follicular thyroid cancer
In rare cases, you may develop an overactive thyroid as a result of thyroid cancer that starts in your thyroid follicles. This can occur if cancer cells in your thyroid gland begin to produce thyroxine or triiodothyronine. This is also known as functioning thyroid cancer.