An overactive thyroid (hyperthyroidism) occurs when your thyroid gland produces too much of the thyroid hormones.
This results in a high level of thyroid hormones called triiodothyronine (also called "T3") and thyroxine (also called "T4") in your body.
The thyroid can become overactive for a number of reasons. Some of the main causes are described below.
In about three in every four cases, an overactive thyroid is caused by a condition called Graves' disease.
This is an autoimmune condition, which means the immune system mistakenly attacks the body. In Graves' disease, it attacks the thyroid and causes it to become overactive.
The cause of Graves' disease is unknown, but it mostly affects young or middle-aged women and it often runs in families. Smoking can also increase your risk of getting it.
Less commonly, an overactive thyroid can occur if lumps called nodules develop on the thyroid.
These nodules are usually non-cancerous (benign), but they contain additional thyroid tissue, which can result in the production of excess thyroid hormones.
It's not known why some people develop thyroid nodules, but they most often affect people over 60 years of age.
An increased level of iodine in the body can cause the thyroid to produce excess thyroid hormones.
This can occasionally occur if you're taking medication that contains iodine, such as amiodarone – a medicine sometimes used to control an irregular heartbeat (arrhythmia).
An overactive thyroid caused by medication will usually improve once the medication is stopped, although it may take several months for your thyroid hormone levels to return to normal.
Other possible causes of an overactive thyroid include:
- high levels of a substance called human chorionic gonadotrophin in the body – this can occur in early pregnancy, a multiple pregnancy or a molar pregnancy (where tissue remains in the womb after an unsuccessful pregnancy)
- a pituitary adenoma – a non-cancerous (benign) tumour of the pituitary gland (a gland located at the base of the brain that can affect the level of hormones produced by the thyroid)
- thyroiditis – inflammation of the thyroid, which can result in extra thyroid hormones being produced
- thyroid cancer – rarely, a cancerous thyroid tumour can affect the production of thyroid hormones