Free T3 blood test
Free triiodothyronine
In short
A typical adult free T3 result is 2 to 4.4 pg/mL. A high free T3 usually goes with an overactive thyroid, or with more thyroid medicine than the body needs. TSH is usually low at the same time. Always read your result against the range printed on your own report.
The unattached form of T3, the most active thyroid hormone.
T3 (triiodothyronine) is the most active thyroid hormone. The thyroid makes some of it, and the body makes the rest from T4. Free T3 is mostly checked for an overactive thyroid, because sometimes it is the only hormone that is high.
Is my Free T3 high or low?
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Typical range
| For | Range | Unit |
|---|---|---|
| Adults | 2–4.4 | pg/mL |
| Adults | 3.1–6.8 | 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 T3 is high
A high free T3 usually goes with an overactive thyroid, or with more thyroid medicine than the body needs. TSH is usually low at the same time.
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
- Less often: a rare growth in the pituitary, the gland that makes TSH
If your Free T3 is low
A low free T3 is common during or after an illness. On its own, it is not a reliable sign of an underactive thyroid.
Common reasons
- A recent illness, surgery or hospital stay, which lowers T3 for a while
- Eating very little or fasting, when the body makes less T3
- Some medicines, including steroid medicines and some for heart rhythm
- An underactive thyroid, usually with a high TSH
- 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 T3 look falsely high on some lab tests, and illness or fasting can lower it.
Questions to take to your appointment
- 1Do my TSH and free T4 point the same way?
- 2Could a recent illness or my medicines explain this?
- 3Is it worth repeating once I feel well?
Read together with
Doctors rarely read Free T3 alone. These usually sit next to it.
Quick answers
What is the free T3 blood test?
Free T3 (Free triiodothyronine): The unattached form of T3, the most active thyroid hormone. T3 (triiodothyronine) is the most active thyroid hormone. The thyroid makes some of it, and the body makes the rest from T4. Free T3 is mostly checked for an overactive thyroid, because sometimes it is the only hormone that is high.
What is a normal free T3 level?
For most adults, free T3 is typically 2 to 4.4 pg/mL. 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 T3 mean?
A high free T3 usually goes with an overactive thyroid, or with more thyroid medicine than the body needs. TSH is usually low at the same time. 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 T3 mean?
A low free T3 is common during or after an illness. On its own, it is not a reliable sign of an underactive thyroid. Common reasons include a recent illness, surgery or hospital stay, which lowers T3 for a while; eating very little or fasting, when the body makes less T3; some medicines, including steroid medicines and some for heart rhythm; and an underactive thyroid, usually with a high TSH.
What can affect my free T3 result?
Biotin, a hair-and-nail supplement, can make free T3 look falsely high on some lab tests, and illness or fasting can lower it.
What should I ask my doctor about my free T3?
Questions worth taking to the appointment: Do my TSH and free T4 point the same way? Could a recent illness or my medicines explain this? Is it worth repeating once I feel well?
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.
Vita: Health Coach on the App StoreSources
- Mayo Clinic Laboratories — T3 (triiodothyronine), free, serum
- MedlinePlus — Triiodothyronine (T3) tests
Stuck on a word? See the medical words. How we choose our sources. More about Vita Health Coach.