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Causes - Overactive thyroid (hyperthyroidism)

An overactive thyroid happens when your thyroid gland produces too much thyroid hormone.

This results in a high level of thyroid hormones called triiodothyronine  and thyroxine  in your body.

The thyroid can become overactive for a many reasons.

Graves' disease

In around 3 in every 4 cases, an overactive thyroid is caused by a condition called Graves' disease.

This is an autoimmune condition. This 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. It mostly affects young or middle-aged women and it often runs in families. Smoking can also increase your risk of getting it.

Thyroid nodules

An overactive thyroid can also occur if lumps called nodules develop on the thyroid.

These nodules are usually benign (non-cancerous), but they contain extra thyroid tissue. This can result in the production of excess thyroid hormones.

It's not known why some people develop thyroid nodules, they mostly affect people people age 60 and older.


An increased level of iodine in the body can cause the thyroid to produce excess thyroid hormones.

This can occur if you're taking medication that contains iodine, such as amiodarone. This is a medicine sometimes used to control an irregular heartbeat (arrhythmia).

An overactive thyroid caused by medication will usually improve when the medication is stopped. But it may take several months for your thyroid hormone levels to return to normal.

Other causes of an overactive thyroid

Other possible causes of an overactive thyroid include:

  • high levels of a substance called human chorionic gonadotrophin in the body - this can happen in early pregnancy, a multiple pregnancy or a molar pregnancy
  • a pituitary adenoma - a non-cancerous tumour of the pituitary gland
  • thyroiditis - inflammation of the thyroid that can result in extra thyroid hormones being produced
  • thyroid cancer - in rare cases a cancerous thyroid tumour can affect the production of thyroid hormones

Diagnosing an overactive thyroid

Content supplied by the NHS and adapted for Ireland by the HSE

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This project has received funding from the Government of Ireland’s Sláintecare Integration Fund 2019 under Grant Agreement Number 123.

Page last reviewed: 14 March 2021
Next review due: 14 March 2024