All you need to know about the thyroid hormone T3 (triiodothyronine)

Thyroid blood results can be hard to decipher. Find out all you need to know about the thyroid biomarker T3.

What is T3?

T3 (also known as triiodothyronine) is a thyroid hormone that plays several important roles in the body, including regulating metabolism and growth. Its levels are regulated by a feedback loop between the pituitary gland, hypothalamus, and thyroid gland. It exists in two forms – free and bound [1].  

Bound T3 vs free T3

When T3 is released by the thyroid gland, most of it binds to proteins in the bloodstream, such as thyroxine-binding globulin (TBG).

T3 that is attached to a protein is known as bound T3. The rest is known as free T3 (FT3), which is the more biologically active form. Together, both forms make up total T3 (TT3) levels. Most labs measure FT3 levels and some measure TT3. Each provides different information about thyroid function.

Around 80% of T3 comes from a process called deiodination. During deiodination, your liver and kidneys convert T4 to T3 [1].

What is reverse T3?

Reverse T3 (rT3) is made from T4. It's chemically similar to T3 but is biologically inactive. Reverse T3 acts as a natural regulator of thyroid function by blocking the action of T3 and inhibiting its effects on cells.

Reverse T3 is not routinely measured in practice. As a standalone test, there is little evidence on how to interpret or manage raised levels. For this reason, many healthcare practitioners don't recommend it. If you do measure rT3, it's important for your doctor to interpret your results alongside other thyroid function tests.

Why do I need to measure my T3 levels?

While T4 levels are often measured in thyroid function tests, they may not provide a complete picture of thyroid activity, as the body's ability to convert T4 into T3 can vary depending on individual factors. T3 can help to give a fuller picture. 

How can I test my T3 levels?

You can test your T3 levels with a simple blood test. Most UK labs test FT3 levels rather than TT3 levels. TT3 levels are sometimes useful when assessing thyroid function, especially if your levels of thyroid hormone-binding proteins are raised. But generally, FT3 is thought to give a more accurate reflection of the amount of thyroid hormone available to cells in your body.

You can test your FT3 level 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 FT3 level to help give a better picture of your overall thyroid health.

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Does the NHS test T3 levels?

The NHS doesn't routinely test T3 levels as part of a thyroid function test. This is because T3 is considered to be less reliable that other thyroid hormones such as TSH and T4.

Your level of T4 reflects the amount of T4 that is able to be converted into T3. Therefore, if T4 levels are normal, it's likely T3 levels are too.

However, in certain situations, such as when a patient's symptoms are not in keeping with their TSH or T4 levels, or when a patient has a history of thyroid disease or thyroid surgery, the doctor may request a T3 level test. Also, private providers often measure T3 levels as part of a more comprehensive thyroid function test.

What is a normal T3 level?

A normal FT3 level differs according to your age. For healthy adults, a normal FT3 range is approximately 3.1–6.8 pmol/L. For TT3, a normal range is approximately 50–160 nmol/L. Note that normal ranges will vary according to the laboratory [2].

What causes a low T3 result?


There are several causes of low T3, including:

  • Hypothyroidism – this could be caused by an autoimmune disease (such as Hashimoto’s thyroiditis), iodine deficiency, or damage to the thyroid gland.
  • Medications – such as steroids, beta-blockers, and anti-seizure medications.
  • Chronic health conditions – such as liver disease, kidney disease, or gut inflammation conditions (such as coeliac and Crohn’s disease).

What causes a raised T3 result?


There are several causes of a raised T3 result, including:

Hyperthyroidism – this could be caused by an autoimmune disease such as Graves’ disease, thyroid nodules or tumours, or inflammation of the thyroid gland.
Incorrect dose of medication – taking too much thyroid hormone replacement can result in high levels of T3.
Pregnancy – your thyroid hormone production increases during pregnancy. If it increases too much, it can lead to high levels of T3.
Liver disease – liver disease can interfere with the production of thyroid hormones, and this can result in higher levels of T3 in the bloodstream.


What symptoms are associated with abnormal FT3 levels?

If you have an abnormal FT3 level for a prolonged period of time, you’re likely to develop symptoms of either hyperthyroidism or hypothyroidism.

If you have a high level of FT3 (often due to hyperthyroidism), your symptoms may include:

  • Anxiety
  • Difficulty concentrating
  • Fatigue
  • Frequent bowel movements
  • Goitre (visibly enlarged thyroid gland) or thyroid nodules

If you have low levels of FT3 (often due to hypothyroidism), your symptoms may include:

  • Tiredness
  • Sensitivity to cold
  • Constipation
  • Dry skin
  • Weight gain

What should I do if my FT3 levels are abnormal?

If your FT3 levels are borderline low but your other thyroid hormone levels are normal, this isn’t always a cause for concern. It could represent a temporary issue that may have resolved on repeating the test at a later date.

However, if your other thyroid function tests are abnormal, it could represent an underlying problem with your thyroid gland, such as an under- or overactive thyroid. Generally, it’s best to discuss any abnormal results with your GP, as you may require monitoring or treatment.

Can I increase my FT3 levels naturally?

Your FT3 levels can be influenced by healthy lifestyle changes. However, whether you can increase your FT3 levels naturally will depend on the cause of your low levels.

Ensuring you eat a thyroid-friendly diet, keep stress levels low, and follow the advice of your medical professional, can help you to keep your FT3 within a healthy range. If your FT3 level is low due to an underlying thyroid condition, you’ll likely require treatment to bring your level back into the normal range.

Where can I get more information about T3 levels and the thyroid?

  • Thyroid Hub – take a look at our Thyroid Hub to find more information on your thyroid health. From information on thyroid disorders to answering questions about whether thyroid conditions can resolve 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 have a diagnosis or not, it offers 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. What is T3? - endocrineweb (no date). Available at: (Accessed: April 19, 2023).
  2. Thyroid Function Tests (no date) NHS choices. NHS. Available at:,ages%200%2D18%20years). (Accessed: April 19, 2023).

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