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Diagnosis - Overactive thyroid (hyperthyroidism)

See your GP if you think you may have an overactive thyroid. A diagnosis will be based on your symptoms and a blood test.

Symptoms of an overactive thyroid

Thyroid function blood test

Your GP may take a sample of blood to check your hormone levels. This is known as a thyroid function test.

It's used to check the levels of:

  • thyroid-stimulating hormone (TSH) - a hormone produced by the pituitary gland that controls the production of thyroid hormones
  • triiodothyronine (T3) - one of the main thyroid hormones
  • thyroxine (T4) - another of the main thyroid hormones

Your levels will be compared to what's normal for a healthy person. A low level of TSH and high levels of T3 and/or T4 usually means you have an overactive thyroid.

What are considered normal levels will vary. It can depend on things such as your age and the testing technique used by the laboratory.

Further tests

If you have high thyroid hormone levels, your GP may refer you to an endocrinologist (specialist in hormone conditions) to find out what's causing it.

Further blood tests

Another blood test may look for anti-thyroid antibodies.

These are usually found if you have Graves' disease. This is a common cause of an overactive thyroid.

A blood test called erythrocyte sedimentation rate may also check for inflammation in your body.

If there are signs of inflammation, it may mean the increase in thyroid hormones is caused by thyroiditis (inflammation of the thyroid).

Thyroid ultrasound

Your GP may arrange a thyroid ultrasound to check if you have any thyroid swelling or nodules.

Thyroid scan

A thyroid scan may be done to look for problems such as lumps (nodules) on your thyroid.

You'll swallow or have an injection of a small amount of a slightly radioactive substance. This will be absorbed by your thyroid.


You cannot have a thyroid scan if you are pregnant or you think you might be pregnant.

A scan shows how much of the substance has been absorbed and the size and shape of your thyroid.

Treatment for 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