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Causes - Overactive thyroid (hyperthyroidism)

Your thyroid is overactive when it produces more hormones than your body needs.

The two main thyroid hormones are:

  • triiodothyronine
  • thyroxine

The thyroid can become overactive for many reasons.

Graves' disease

About 3 in 4 people with an overactive thyroid have a condition called Graves' disease.

Graves' disease is an autoimmune condition. This means the immune system attacks the body by mistake. It damages the thyroid and causes it to become overactive.

The cause of Graves' disease is unknown, but there are risk factors.

You have a higher risk of getting Graves' disease if you:

  • are a woman age 20 to 50
  • have a family history of Graves' disease
  • smoke

Thyroid nodules

Lumps (nodules) can sometimes develop on your thyroid gland. This can cause it to become overactive.

The nodules are usually benign (non-cancerous). They may contain thyroid tissue that can make extra hormones.

What causes thyroid nodules is not clear. You are more likely to get them if you are 60 or older.


Iodine can cause the thyroid gland to produce more hormones than your body needs. There is iodine in some medicines - for example, amiodarone. Amiodarone can be used to control an irregular heartbeat (arrhythmia).

Symptoms of an overactive thyroid will usually improve when you stop using the medicine. But it can take months for your hormone levels to return to normal.

Other causes of an overactive thyroid

Other possible causes of an overactive thyroid include:

  • high levels of a hormone called human chorionic gonadotropin - 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
  • thyroid cancer (rarely)

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: 20 December 2023
Next review due: 20 December 2026