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Diagnosis - Underactive thyroid (hypothyroidism)

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 a heart attack.

Therefore, you should see a 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.

Doctors may refer to this as "free" T4 (FT4).

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.

The 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 is not routinely offered.

Less commonly, a thyroid antibody test may be recommended after a thyroid function test. This is to help diagnose or rule out autoimmune thyroid conditions, such as Hashimoto's thyroiditis. A thyroid antibody test is only likely to be recommended if the GP suspects you have an autoimmune thyroid condition.

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


A 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 medicine
  • are taking a medicine known to cause a reduction in thyroid hormones, such as amiodarone or lithium

Page last reviewed: 10 May 2021
Next review due: 10 May 2024