Diagnosing underactive thyroid 

It's very important that an underactive thyroid (hypothyroidism) is diagnosed as soon as possible.

Low levels of thyroid-producing hormones, such as triiodothyronine (T3) and thyroxine (T4), can change the way the body processes fat.

This can cause high cholesterol and atherosclerosis (clogging of the arteries), which can potentially lead to serious heart-related problems, such as angina and heart attack.

Therefore, you should see your GP and ask for a blood test if you repeatedly have symptoms of an underactive thyroid.

Thyroid function test

A blood test measuring your hormone levels is the only accurate way to find out whether there's 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 results show raised TSH but normal T4, you may be at risk of developing an underactive thyroid in the future.

Your GP may recommend that you have a repeat blood test every so often to see whether you eventually develop an underactive thyroid.

Blood tests are also sometimes used for other measurements, such as checking the level of a hormone called triiodothyronine (T3). However, this isn't routine, because T3 levels can often remain normal, even if you have a significantly underactive thyroid.

The Lab Tests Online UK website has more information about the different types of thyroid function tests.


Your GP may refer you to an endocrinologist (a specialist in hormone disorders) 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 a medication known to cause a reduction in thyroid hormones, such as amiodarone or lithium

Page last reviewed: 11/05/2015

Next review due: 11/05/2017