Find out about the main tests used to diagnose an overactive thyroid (hyperthyroidism), including a thyroid function blood test.

See your GP if you think you may have an overactive thyroid (hyperthyroidism).

A diagnosis will be based on your symptoms and the results of a blood test that assesses how well your thyroid is working.

Thyroid function blood test

Your GP may take a sample of blood and send it to a laboratory 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 (a gland at the base of the brain) 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 varies depending on things such as your age and the exact testing technique used by the laboratory.

Further tests

If you have high thyroid hormone levels, you may be referred to a specialist for the tests below to find out what's causing it.

Further blood tests

Another blood test may be carried out to look for anti-thyroid antibodies.

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

A blood test called erythrocyte sedimentation rate (ESR) may also be done to 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 scan

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

For the test, you'll be asked to swallow or have an injection of a small amount of a slightly radioactive substance that will be absorbed by your thyroid.

A scan is then carried out to see how much of the substance has been absorbed and to examine the size and shape of your thyroid.

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