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Free T4 is the active form of the hormone thyroxine in your blood. A level that is too low or too high can point to a problem with your thyroid.[1]
On this page you can read what free T4 is, what the guidelines say about a low or high level, what affects your level and which values are best to look at alongside it.
Also known as: free thyroxine, free T4, thyroxine. At the lab: FT4.
Your thyroid is a small, butterfly-shaped gland at the front of your neck. The main hormone it makes is called thyroxine, or T4.[1]
T4 is found in your blood in two forms. Free T4 is the active form that enters your tissues. Bound T4 is attached to proteins and stays in your blood as a reserve until your tissues need it.[1]
A free T4 test measures only the free part. Medical experts consider this test more accurate than a total T4 test, which is why it is used more often.[1]
Thyroid hormones control how your body uses energy. They act on almost every organ. They affect your breathing, your heart rate, your weight, your digestion and your mood. In children, they also affect growth.[1]
Too little thyroid hormone slows your body down. For example, you may feel tired, your weight may go up, you may feel cold easily and your skin may become dry.[1]
Too much thyroid hormone speeds your body up. For example, your weight may drop even though you eat more than usual, your heart may beat fast or irregularly, and you may feel nervous or sleep badly.[1]
Symptoms differ from person to person. Thyroid problems often run in families, and that can be a reason to have your level measured.[1]
The lab BioProfile works with, Labplusarts, uses a reference range of 11.0 to 21.0 pmol/l for free T4. The lab calls a level between these limits normal.[2]
| Value | What it can mean | Who it is for |
|---|---|---|
| 11.0 to 21.0 pmol/l[2] | The lab calls a value between these limits normal | Results measured by Labplusarts |
Source: Labplusarts, the lab BioProfile works with. Reference range for free T4 (FT4).[2]
A level below normal can point to a thyroid that is working too slowly. It can also point to an immune reaction in which your immune system attacks the thyroid, to certain stages of an inflamed thyroid, or to too much or too little iodine in your diet. Sometimes the pituitary gland is the cause, but that is rare. In a newborn baby, it can point to a thyroid that has worked too slowly since birth. A low level can also come from thyroid treatment, such as radiation, radioactive iodine or surgery.[1]
A level above normal can point to a thyroid that is working too hard. It can also point to an immune reaction in which your immune system attacks the thyroid, to certain stages of an inflamed thyroid, or to an enlarged thyroid or a lump that makes too much hormone. Your doctor usually looks at your result alongside other thyroid values to see what it means.[1]
BioProfile is working on its own zones for each value, from risk to optimal. A doctor reviews those zones before they appear in your result.
Some medicines and supplements can affect your result. Tell your healthcare provider what you take. Do not stop taking a medicine without talking to them first.[1]
Do you take thyroid hormone because your thyroid works too slowly? Then a T4 test can show how well that medicine is working.[1]
Too much or too little iodine in your diet can go together with a low T4.[1]
An abnormal T4 usually comes from the thyroid itself. Sometimes the cause lies in the pituitary gland, which makes too much or too little TSH, or in the hypothalamus, the part of your brain that controls the pituitary gland.[1]
Free T4 is included in BioProfile 87, 121 and 150.[3]
The BioProfile editorial team wrote this page on 10 October 2026 from the sources below, which we read on 10 October 2026. Every number on this page appears in one of those sources, with that source's number next to it. An expert has yet to check this page; after that check, the expert's name and the date of the check will appear at the top. This page offers explanation, not a diagnosis. Discuss a value outside the range with your GP.
