Free T4

What is free T4 (FT4)?

T4 (Thyroxine) is a thyroid hormone that is produced by the thyroid gland and plays an important role in the body’s metabolism. Nearly all the T4 and T3 found in the blood is bound to protein. The rest is free (unbound) and is the biologically active form of the hormone. A free T4 test measures the amount of free T4 in the blood.

Why test free T4?

Unless this is ordered as a retest or for a specific reason, we recommend ordering this as part of a health test. Measuring FT4 is important because it is an indicator of thyroxine activity in the body. Specific reasons include:

Health check
Discover your risk factors for lifestyle diseases in time to do something about them.  Free T4 is an important part of any thyroid profile.

Including depression, weakness, fatigue, sleeping problems, sensitivity to heat/cold, dry skin, hair loss, increased heart rate, and infertility.

Monitor an existing thyroid condition as you make dietary and lifestyle changes or take medication.  We can send any results to your doctor.

For a more comprehensive thyroid test.

What do the results mean?

Free T4 results need to be interpreted in the context of other test results.

What causes low free T4?

The level of free T4 in your bloodstream is lower than expected. Free T4 results are interpreted in the context of other test results and your current symptoms. Low free T4 indicates that the thyroid does not produce enough thyroid hormone (hypothyroidism). In hypothyroidism, TSH is normally high and T3 and T4 are normal to low. Low T4 may indicate iodine deficiency.

Causes of low thyroid hormones include autoimmune thyroid disease (most often Hashimoto’s thyroiditis), chronic inflammation and oxidative stress often caused by lifestyle and diet, calorie restriction, low estrogen, low testosterone, and some nutrient deficiencies including iodine, selenium, zinc, iron, chromium, copper, vitamin A, and B vitamins.  

There is some evidence that eating a very low-carb or ketogenic diet may affect and lower thyroid function. The conversion of T4 to the active form of T4 is dependent on insulin. If you notice that you suddenly develop hypothyroid symptoms when sticking to a very low carbohydrate diet, you most likely need to increase your carbohydrate intake. We recommend you stick to healthy sources of carbohydrates such as whole grains (preferably gluten-free), beans, lentils, root vegetables, pumpkin, squash, fruit, and berries. 

Common symptoms of hypothyroidism (underactive thyroid) include fatigue, feeling cold, often cold hands, feet, and the tip of your nose, constipation, dry skin, weight gain/difficulty losing weight, puffy face, hoarseness, muscle weakness, stiff and achy muscles, slowed heart rate, dry and thinning hair, poor memory, and depression.

What causes high free T4?
Free T4 results need to be interpreted in the context of other test results and your current symptoms. In hyperthyroidism. TSH levels are often normal to low and T3 and T4 are normal to high. High free T4 can be caused by Graves’ disease (an autoimmune disorder that causes hyperthyroidism), thyroid disease, high iodine intake, and too much thyroid replacement medication.

More information

Thyroid hormones play an important role in the body's metabolism. T4 (thyroxin) is an important hormone that is produced by the thyroid gland and transported by the blood to the body's cells.  T4 can be free (unbound) or bound to protein in your blood.  Nowadays most health providers consider free T4 to be the most important test. Ideally, it should be measured together with TSH and T3.

If the thyroid gland produces too much hormone, symptoms are those of a raised metabolism, including rapid heart rate, sweating, increased appetite, weight loss, tremors, and poor sleep.  Low thyroid hormone has the opposite effect, with side effects such as feeling cold, fatigue, weight gain, hair loss and constipation and the other symptoms above.

TSH stimulates the production and release of T4 (primarily) and T3 from the thyroid gland. Thyroid health is somewhat complex and your practitioner should take into account the following:

Blood test results: Free T4, free T3 and TSH. Do you produce enough thyroid hormone? Is your T4 converting properly to T3? Is your TSH within an optimal range?

Symptoms: If the above are within normal range, but you have symptoms, or are pregnant, thyroid antibody testing should be undertaken.

Individual differences and your results: Do you have symptoms and your results are low or high normal? Perhaps these levels are too low or high for you.


No advance preparation is needed for this test.

On the day of the test

Do not take your thyroid medication until after you have taken this test.

Remember to take I.D. with you when going to take a test.

Do not work out before going to do this test, as this may affect your results

Take this test before 10 am, as results can vary throughout the day.  If you are repeating a test, try to take it at the same time of day.

Arrive in good time and sit down quietly for 15 minutes before the test is taken.


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