Hyperthyroidism occurs when your thyroid gland produces too much of one of the thyroid hormones - thyroxine and triiodothyronine. Excessive production 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 hyperthyroidism. It can run in families, and can occur at any age, although it is most common in women between 20-40 years of age. You are also more likely to develop Graves' disease if you smoke.

Graves' disease is an autoimmune condition, which means that it occurs when your immune system mistakes a healthy substance in your body for a toxic substance and attacks it. Normally, your immune system makes antibodies which attack bacteria and fight infection. However, in autoimmune conditions, the antibodies attack your body's 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' opthalmopathy. You may find that your eyes 'stand 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, which are known as nodules. It is not known why these 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 hyperthyroidism. Nodules that contain abnormal thyroid tissue are described as toxic.

Toxic multinodular goiter 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

The iodine that is contained in the food that 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 hyperthyroidism which can result from this is known as iodine-induced hyperthyroidism, which is 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 hyperthyroidism 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, on its own.