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Thyroid, underactive - Diagnosing underactive thyroid

NHS Choices Medical Reference

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If you have symptoms of an underactive thyroid

(hypothyroidism), see your GP and ask for a blood test.

blood test measuring your hormone levels is the only accurate way to find out if there is a problem.

The test, called a thyroid function test, looks at levels of thyroid-stimulating hormone (TSH) and thyroxine (T4) in the blood.

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

If your test shows raised TSH but normal T4, it means you may be at risk of developing an underactive thyroid in the future.

Your GP may advise a repeat blood test every so often to see if you eventually develop an underactive thyroid.

Blood tests may sometimes be used for other measurements, such as checking the level of a hormone called triiodothyronine (T3). However, this is not routine because levels of T3 can remain normal even if you have a significantly underactive thyroid.

For more information on testing, go to Lab Tests Online: thyroid function tests.


Your GP may refer you to a specialist in hormone disorders, known as an endocrinologist, if you:

  • are younger than 16
  • are pregnant or trying to get pregnant
  • have just given birth
  • have another health condition, such as heart disease, which may complicate your medication
  • are taking amiodarone or lithium medication
Medical Review: June 30, 2013
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