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Diagnosis - Overactive thyroid (hyperthyroidism)

Contact your GP if you think you may have an overactive thyroid. They may ask about your symptoms and do a blood test.

Thyroid function blood test

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

It checks the levels of:

  • thyroid-stimulating hormone (TSH) - made by the pituitary gland and controls the production of thyroid hormones
  • triiodothyronine (T3) - made by the thyroid gland
  • thyroxine (T4) - made by the thyroid gland

A low level of TSH and high levels of T3 or T4 usually mean you have an overactive thyroid.

Your doctor will check your levels of these hormones against the normal range for your age and sex.

Further tests

If you have high thyroid hormone levels, your GP may refer you to an endocrinologist (specialist in hormone conditions) to find out the cause.

Other blood tests

Another blood test may look for anti-thyroid antibodies. These antibodies may be in your blood if you have Graves' disease. This is a common cause of an overactive thyroid.

They may also check your erythrocyte sedimentation rate. This blood test shows if there is inflammation in your body. If there are signs of inflammation, you may have thyroiditis (inflammation of the thyroid). Thyroiditis can cause an increase in thyroid hormone levels.

Thyroid ultrasound

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

Thyroid scan

You may need a thyroid scan to check for lumps (nodules) on your thyroid.


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

During the scan the tester will:

  1. give you a slightly radioactive substance - you may swallow this or have an injection
  2. check how your thyroid absorbs the substance
  3. check 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: 20 December 2023
Next review due: 20 December 2026