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Diagnosis - Underactive thyroid (hypothyroidism)

It's important to diagnose an underactive thyroid as soon as possible.

Low levels of thyroid hormones can change the way the body processes fat.

This can cause high cholesterol and atherosclerosis (clogging of the arteries).

This could lead to serious heart-related problems, such as angina and a heart attack.

When to see your GP

Talk to your GP if you have symptoms of an underactive thyroid. Ask for a thyroid function blood test.

If you are diagnosed with an underactive thyroid, you may need repeat blood tests to maintain your hormones at the correct levels.

Symptoms of an underactive thyroid gland

Thyroid function test

Your GP will do a blood test. A lab will check the hormone levels in your blood.

The test looks at blood levels of:

  • thyroid-stimulating hormone (TSH)
  • thyroxine (T4)

A high level of TSH and a low level of T4 in the blood could mean you have an underactive thyroid.

If your test results show raised TSH but normal T4, you may be at risk of developing a problem in the future.

Your GP may advise you to have a blood test every so often. This is to see if you will develop an underactive thyroid.

Other test indicators

Blood tests are also sometimes used for other measurements.  Your GP may check the level of a hormone called triiodothyronine (T3). But this is not routine.

Thyroid antibody test

Your GP may advise a thyroid antibody test. This is to help diagnose or rule out conditions such as Hashimoto’s thyroiditis. You'll usually only have this test if your GP thinks you have an autoimmune thyroid condition. It's not a common test.


Your GP may refer you to an endocrinologist. This is a specialist in hormone disorders.

You may be referred if you:

  • are younger than 16
  • are pregnant or trying to get pregnant
  • have just given birth
  • have another health condition that may complicate your medicine, such as heart disease
  • are taking medicine that causes a reduction in thyroid hormones, for example, amiodarone or lithium

Find out more about an underactive 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: 25 March 2021
Next review due: 25 March 2024