All you need to know about thyroxine (T4)

Thyroid blood results can be hard to decipher. Find out all you need to know about the thyroid hormone T4 and why doctors don’t always routinely test for it.

What is T4?

T4, or thyroxine, is one of two major hormones produced by the thyroid gland. It is secreted in the largely inactive form and then converted into T3 (the active form) [1] and plays an important role in several processes in the body.

T4 plays a role in:

  • Regulating metabolism
  • Growth and development
  • Regulation of body temperature
  • Control of heart rate and blood pressure
  • Muscle strength, tone, and repair
  • Bone health
  • Mood
  • Memory and concentration
  • Digestion


What is free T4?

Almost all T4 (over 99%) travels in the bloodstream bound to a protein – this is known as bound T4. The rest is known as free T4 (FT4) which is able to act directly on the tissues. FT4 tends to better reflect thyroid function compared to total T4, which is why it tends to be the favoured measurement [2].

Why do I need an FT4 blood test?

An FT4 blood test (also known as a total thyroxine test or a serum thyroxine test), may be ordered by your GP for several reasons, including:

  1. To evaluate thyroid function – a FT4 blood test is often used as part of a thyroid function test to evaluate the overall health of the thyroid gland. If the thyroid gland is not producing enough FT4, it can indicate hypothyroidism, whereas too much FT4 can indicate hyperthyroidism.
  2. To monitor the effects of treatment
  3. To investigate symptoms such as fatigue, weight changes, or changes in heart rate.
  4. To screen for thyroid disorders – especially if someone has a family history of thyroid disease or an increased risk of developing a thyroid condition.

How can I test my FT4 levels?

You can test your FT4 levels from the comfort of your home, with an at-home blood test such as our Thyroid Function Blood Test. Our range of Thyroid Blood Tests check your FT4 levels to help give a better picture of your overall thyroid health.


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Does the NHS test FT4 levels?

The NHS does test FT4 levels, but often not routinely. The first line test is TSH and then they will test FT4 and/or FT3 if necessary [3].

What is a normal level of T4?

A normal FT4 level is between 12 and 22 pmol/L, but these figures will vary according to the laboratory. However, during pregnancy these levels may change.

What is a normal T4 level in pregnancy?

The table below acts as a guide for normal FT4 levels in pregnancy [3]. Note that ranges will vary according to the laboratory.

T4 levels throughout pregnancy table


What causes a low T4 level?


There are several conditions that could cause a low FT4 level. The most common causes of a low FT4 level are:

  1. Autoimmune conditions – Hashimoto’s thyroiditis is one of the most common causes of an underactive thyroid.
  2. Previous thyroid treatment – such as thyroid surgery or radioactive iodine therapy  [4]
  3. Medications – drug-induced hypothyroidism could be the cause of a low FT4 level.

There are also other causes, such as a problem with the pituitary gland, radiation therapy (such as in the case of treatment of thyroid cancer), and congenital hypothyroidism (present from birth). FT4 levels also tend to decline with age.

Do FT4 levels drop with age?

There is evidence to suggest that FT4 levels drop with age [5]. Being over the age of 50 is a risk factor in developing a thyroid condition. For many, thyroid disorders are more likely to go undiagnosed as you are older as other conditions may mask your thyroid conditions, such as menopause.

What causes a raised FT4 level?

There are several causes of a raised FT4 level, ranging from conditions to lifestyle factors.

Causes of a raised FT4 level include:

  1. Autoimmune conditions – Graves' disease is one of the most common causes of an overactive thyroid
  2. Thyroid nodules – the tissue in thyroid nodules can create excess thyroid hormones.
  3. Excessive iodine intake – an increased level of iodine can cause the thyroid to produce excess FT4 [6].
  4. Medications – taking too much levothyroxine (used to treat hypothyroidism) can lead to raised FT4 levels.

What symptoms are associated with an abnormal FT4 level?

If you have an abnormal level of FT4, you may display a range of symptoms. Your symptoms will depend on whether you have a high level or a low level.

We have outlined some symptoms associated with an abnormal FT4 level in the table below.

Symptoms of low and high t4 levels table


What should I do if my FT4 level is abnormal?

This will depend on the rest of your thyroid results. Generally, if your TSH is normal and your FT4 level is slightly outside the normal range, this is not a cause for concern. Your doctor may suggest repeating the test in a few months’ time, by which point, levels may have returned to normal. Occasionally, it could be an early sign of a thyroid condition.

If both your TSH and FT4 results are abnormal or you have associated symptoms, this could mean you have an underlying thyroid condition. And if you’re already established on treatment, it may mean your dose of thyroid medication needs altering.

It’s best to speak to your GP about any abnormal results to ask about any further tests that may be needed.

Want to know more about managing a thyroid condition? Read more in our blog: ways to manage your thyroid condition.

Where can I get more information about T4 levels and the thyroid? 

  • Thyroid Hub – from information on thyroid disorders to answering questions about whether thyroid conditions can go away by themselves, our Thyroid Hub is here to support you.
  • Thyroid UK – Thyroid UK is a charity that seeks to help you through your thyroid journey. Whether you are undiagnosed or diagnosed, they offer plenty of support and information.
  • British Thyroid Foundation – BTF is a charity that helps people who have been diagnosed and are living with a thyroid disorder. And it offers support based on patient experience.


  1. Thyroxine (no date) You and Your Hormones. Available at: (Accessed: April 19, 2023).
  2. Thyroid function tests - american thyroid association (no date). Available at: (Accessed: April 19, 2023).
  3. Thyroid Function Tests (no date) NHS choices. NHS. Available at:,are%20then%20performed%20as%20necessary. (Accessed: April 19, 2023).
  4. Causes of hypothyroidism (no date) NHS choices. NHS. Available at: (Accessed: April 19, 2023).
  5. Aggarwal, N. and Razvi, S. (2013) “Thyroid and aging or the aging thyroid? an evidence-based analysis of the literature,” Journal of Thyroid Research, 2013, pp. 1–8. Available at:
  6. Causes of hyperthyroidism (no date) NHS choices. NHS. Available at:,an%20irregular%20heartbeat%20(arrhythmia) (Accessed: April 19, 2023). 

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