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
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 a healthy substance in the body for a toxic substance and attacks it. Normally, your immune system makes antibodies (proteins) to attack bacteria and fight infection. However, in autoimmune conditions, the antibodies attack healthy tissues instead. It is not known what triggers the immune system to do this.
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 that your eyes bulge out, or appear more prominent.
It is not fully understood why the eyes are affected by Graves' disease, but it is thought that antibodies from your immune system may also affect the tissues around the eye.
Thyroid nodules
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 benign (non-cancerous).
However, the nodules can contain abnormal thyroid tissue, which can affect the normal production of thyroxine or triiodothyronine, causing overactive thyroid. Nodules that contain abnormal thyroid tissue are described as toxic.
Toxic multinodular goitre is the second most common cause of hyperthyroidism after Graves' disease, and occurs when there are two or more nodules in your thyroid gland.
However, if you only have a single nodule in your thyroid gland, it is known as a toxic thyroid nodule or adenoma. Toxic thyroid nodules account for about 5% of cases of hyperthyroidism.
Iodine supplements
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.
The type of overactive thyroid that can result from this is known as iodine-induced hyperthyroidism, sometimes referred to as Jod-Basedow phenomenon. It usually only occurs if you already have non-toxic nodules in your thyroid gland.
Amiodarone
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. This type of hyperthyroidism is called amiodarone-induced hyperthyroidism.
Follicular thyroid cancer
In rare cases, you may develop overactive thyroid as a result of thyroid cancer that starts in your thyroid follicles. This can occur if the cancer cells in your thyroid gland begin to produce thyroxine or triiodothyronine.
