Free T4 blood test
Free thyroxine
In short
A typical adult free T4 result is 0.9 to 1.7 ng/dL. A high free T4 usually goes with an overactive thyroid, or with more thyroid medicine than the body needs. TSH is usually low when free T4 is truly high. Always read your result against the range printed on your own report.
The active, unattached form of the thyroid's main hormone.
Thyroxine (T4) is the main hormone the thyroid releases. Most of it rides on proteins in the blood; free T4 is the unattached part the body can use. Doctors read it with TSH to see how the thyroid is working.
Is my Free T4 high or low?
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Typical range
| For | Range | Unit |
|---|---|---|
| Adults | 0.9–1.7 | ng/dL |
| Adults | 12–22 | pmol/L |
Typical ranges for adults. Labs differ a little, so the range printed next to your result is the one to go by.
If your Free T4 is high
A high free T4 usually goes with an overactive thyroid, or with more thyroid medicine than the body needs. TSH is usually low when free T4 is truly high.
Common reasons
- For people on thyroid medicine, a dose above what the body needs
- Graves' disease, an immune condition that makes the thyroid overactive
- Thyroid nodules, small lumps that make extra hormone on their own
- Thyroiditis, an inflamed thyroid that leaks stored hormone for a few weeks
- Too much iodine, for example from supplements or contrast dye for scans
- Less often: a rare growth in the pituitary, the gland that makes TSH
If your Free T4 is low
A low free T4 usually goes with an underactive thyroid, and TSH is then usually high. A serious illness can also lower it for a while.
Common reasons
- A recent serious illness, which can lower thyroid tests for a while
- Hashimoto's, a common immune condition that gradually slows the thyroid
- For people on thyroid medicine, a dose below what the body needs
- Past thyroid surgery or other treatment for an overactive thyroid
- Some medicines, including steroid medicines and some seizure medicines
- Less often: a pituitary problem, when TSH is not raised as expected
What can throw it off
Biotin, a hair-and-nail supplement, can make free T4 look falsely high on some lab tests, and a serious illness can shift it for a while.
Questions to take to your appointment
- 1What does my TSH show next to this?
- 2Could my medicines, biotin or a recent illness explain it?
- 3Is it worth repeating along with TSH?
Read together with
Doctors rarely read Free T4 alone. These usually sit next to it.
Quick answers
What is the free T4 blood test?
Free T4 (Free thyroxine): The active, unattached form of the thyroid's main hormone. Thyroxine (T4) is the main hormone the thyroid releases. Most of it rides on proteins in the blood; free T4 is the unattached part the body can use. Doctors read it with TSH to see how the thyroid is working.
What is a normal free T4 level?
For most adults, free T4 is typically 0.9 to 1.7 ng/dL. Some countries print it in pmol/L, so check the unit first. Labs differ a little, so the range printed next to your result is the one to go by.
What does a high free T4 mean?
A high free T4 usually goes with an overactive thyroid, or with more thyroid medicine than the body needs. TSH is usually low when free T4 is truly high. Common reasons include for people on thyroid medicine, a dose above what the body needs; Graves' disease, an immune condition that makes the thyroid overactive; thyroid nodules, small lumps that make extra hormone on their own; and thyroiditis, an inflamed thyroid that leaks stored hormone for a few weeks. A doctor reads it together with your other results and how you feel.
What does a low free T4 mean?
A low free T4 usually goes with an underactive thyroid, and TSH is then usually high. A serious illness can also lower it for a while. Common reasons include a recent serious illness, which can lower thyroid tests for a while; Hashimoto's, a common immune condition that gradually slows the thyroid; for people on thyroid medicine, a dose below what the body needs; and past thyroid surgery or other treatment for an overactive thyroid.
What can affect my free T4 result?
Biotin, a hair-and-nail supplement, can make free T4 look falsely high on some lab tests, and a serious illness can shift it for a while.
What should I ask my doctor about my free T4?
Questions worth taking to the appointment: What does my TSH show next to this? Could my medicines, biotin or a recent illness explain it? Is it worth repeating along with TSH?
What this page can’t do
It reads each number on its own.
Doctors read results together — ALT with AST, ferritin with hemoglobin, TSH with free T4. A pattern says more than any single flag.
It doesn't know you.
Your age, your medicines, how you feel and your last result all change what a number means.
It doesn't remember.
Close this page and your results are gone. That's on purpose: nothing was ever sent to us.
We also make Vita: Health Coach, an app for those parts. It reads your whole report with your medicines and history in mind, answers follow-up questions in plain words, and keeps your reports in one place — for you and for the people you look after.
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