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Free T4 Blood Test

Written by Ada’s Medical Knowledge Team

Updated on

A free T4 blood test can be used to assess the function of your thyroid and to diagnose possible medical conditions. The thyroid is a gland which is situated in the front part of the neck. It produces vital hormones called T3 and T4, which regulate cell metabolism. Some medical conditions can cause the thyroid to produce too much or too little of these thyroid hormones, which can lead to a number of symptoms.

What is a free T4 blood test?

A T4 free blood test is used by doctors to check the function of the thyroid gland, which is a small gland located at the front of the neck below the larynx (upper opening of the windpipe) that is important for the regulation of our metabolism. There are a few hormones that are important for the function of the thyroid gland, namely: 1

  • TSH (thyroid stimulating hormone), produced by the pituitary gland
  • T3 (triiodothyronine), produced by the thyroid gland
  • T4 (thyroxine), produced by the thyroid gland

T3 is the more active form of thyroid hormone, whereas T4 is generally an inactive form which gets converted by the body into active thyroid hormone. 

When the thyroid is active and the amount of T3 and T4 in the blood increases, this gives important feedback to the pituitary gland to decrease the amount of TSH production. As a result, the thyroid will slow the production of thyroid hormones down. In this case, the decreased amount of T3 and T4 in the blood gives signals to the pituitary to produce more TSH, which stimulates the thyroid to produce more thyroid hormones again. This important feedback system is normal and makes sure that the body remains balanced. 

In order for the thyroid to function properly, it’s important that T3, T4 and TSH work well together. When there’s an imbalance between these hormones, you may experience symptoms of an overly active or underactive thyroid. A free T4 test can be used to have a look at the function of the thyroid. This way, it can help determine whether or not a thyroid condition is present. 1 2 3

Is a T4 blood test and free T4 blood test the same?

When a problem with the thyroid is suspected, your doctor may do some thyroid function tests. These tests are done through blood work, and usually consist of a TSH, free T4 and free T3 test. TSH is usually tested first, as changes in the values of this hormone will become noticeable first.

If a change in TSH is noticed, the amount of T3 and T4 will be tested as well. As T3 and T4 exist in a bound form (bound to a protein) and in a free form (not bound to anything), there are multiple test options available. A total T4 test can measure the total amount of both free and bound T4 in the blood, while a free T4 test will only measure the amount of T4 in its free form. 

The free T4 blood test is used most often to monitor T4, as it’s the free form of T4 that can have an impact on the body. The test is done by subtracting a sample of blood from a superficial vein in the arm. For this process, a small needle is used. Afterwards, the blood sample will be sent to a laboratory where the amount of T4 will be measured. 2 3 4 5

Who should have T4 levels tested?

There are certain conditions of the thyroid and the pituitary gland that can cause an imbalance in the hormonal system, leading to an overly active thyroid gland (hyperthyroidism) or an underactive thyroid gland (hypothyroidism). These conditions can cause a variety of symptoms, and can be diagnosed with a blood test. If you’re experiencing symptoms that might indicate hypo-or hyperthyroidism, then it’s useful to have your free T4 tested. 

If you’re already being treated for a thyroid condition, then it’s also useful to get regular check-ups of your thyroid function, to make sure that your condition is under control. 5

The results of your free T4 blood test

The results of your free T4 blood test are usually looked at together with the results of other thyroid function tests like the TSH and free T3 test. It’s also very common for your doctor to check your thyroid antibodies, such as your thyroid peroxidase antibodies (TPOAb), thyroglobulin antibodies (TgAb), and thyroid stimulating hormone receptor antibodies (TSHR Ab) if an autoimmune thyroid condition is suspected. 6

High free T4 might indicate that you have hyperthyroidism. This causes symptoms such as: 3 7

  • Anxiety and irritability
  • Intolerance of heat
  • Fatigue
  • Swelling of the thyroid gland
  • Racing heart rate
  • Weight loss
  • Reduced libido

Low free T4 might indicate hypothyroidism. This causes symptoms such as: 8

  • Tiredness
  • Sensitivity to cold
  • Weight gain 
  • Depression and slow thoughts
  • Muscle cramps
  • Dry skin, brittle hair and nails
  • Reduced libido

How often should free T4 be tested?

A free T4 blood test should be done when your thyroid isn’t functioning properly. Usually, this goes together with symptoms of hyper- or hypothyroidism. If you are being treated for a thyroid condition, you may also get your free T4 tested regularly to monitor how well your thyroid is responding to treatment. If thyroid conditions run in your family, then your doctor might also check your thyroid hormones more often. 2

Free T4 test FAQs

Q: Where can I test my free T4 levels?

Your free T4 levels can be tested by your general practitioner or with an at-home test kit.

Q: What is the normal range for free T4?

Your FT4 values should be between 12.0 - 22.0 pmol/L.


  1. Armstrong M. et al. (2022). Physiology, Thyroid Function. Accessed on July 11, 2022.

  2. Lab tests online UK (2018). FT4. Accessed on July 11, 2022.

  3. NHS (2021). Thyroid Function Tests (TSH, FT4, FT3. Accessed on July 11, 2022.

  4. Lab tests online UK (2019). Thyroid Function Tests. Accessed on July 10, 2022.

  5. NHS (2019). Diagnosis overactive thyroid (hyperthyroidism). Accessed on July 10, 2022.

  6. Lab tests online UK (2018). Thyroid antibodies. Accessed on July 11, 2022.

  7. NHS (2019). Symptoms overactive thyroid (hyperthyroidism). Accessed on July 10, 2022.

  8. NHS (2021). Symptoms underactive thyroid (hypothyroidism). Accessed on July 11, 2022.