About 3 in every 4 people with an overactive thyroid gland have a condition called Graves' disease.
Graves' disease is an autoimmune condition where your immune system mistakenly attacks your thyroid which causes it to become overactive.
The cause of Graves' disease is unknown, but it mostly affects young or middle-aged women and often runs in families. Smoking can also increase your risk of getting it.
Less commonly, your thyroid can become overactive if lumps (nodules) develop on your thyroid.
Nodules are usually non-cancerous (benign), but they may contain thyroid tissue, which can result in the production of excess thyroid hormones.
It's not known why some people develop thyroid nodules, but they usually affect people over 60 years of age.
An increased level of iodine in your body can cause your thyroid to produce excess thyroid hormones.
This can occasionally happen if you're taking medicine that contains iodine, such as amiodarone, which is sometimes used to control an irregular heartbeat (arrhythmia).
An overactive thyroid that's caused by a medicine will usually improve once you stop taking that medicine, 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 your body – this can happen in early pregnancy, a multiple pregnancy or a molar pregnancy (where a lump of abnormal cells grows in the womb instead of a healthy foetus)
- a pituitary adenoma – a non-cancerous (benign) tumour in the pituitary gland (a gland at the base of the brain that can affect the level of hormones produced by your thyroid)
- thyroiditis – swelling (inflammation) of your thyroid, which can cause extra thyroid hormones to be produced
- thyroid cancer – rarely, a cancerous thyroid tumour can affect the production of thyroid hormones