If you have an overactive thyroid (hyperthyroidism), your thyroid gland produces too much of the thyroid hormones.
This results in high levels of the 2 main thyroid hormones, triiodothyronine (also called T3) and thyroxine (also called T4) in your body.
There are a number of conditions that can cause your thyroid to become overactive.
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: