The short answer
Free T3 is the active form of thyroid hormone, the one your cells respond to. Its main use is checking for an overactive thyroid, where it can rise before free T4 does. Labcorp’s adult range is 2.0 to 4.4 pg/mL.1,4
- If yours is high
- With a low TSH, you have too much thyroid hormone, from an overactive thyroid, an inflamed thyroid or too high a dose of thyroid hormone, so see a clinician soon. With a TSH that isn’t low, check the test first.4
Check your result
Type the number from your report. It stays on this page.
- Low Under 2.0
- In range 2.0 to 4.4
- High 4.5 or more
- Low, TSH low Under 2.0
- Normal, TSH low 2.0 to 4.4
- Overactive 4.5 or more
- Low Under 2.0
- In range 2.0 to 4.4
- High, TSH not low 4.5 or more
- TSH high Under 4.5
- High, TSH not low 4.5 or more
Enter your result to see what it means and what to do next.
Got a result from another lab? Ask Picardo about it. The app explains your results, and a physician can follow up.
The free T3 cutoffs are Labcorp’s adult range.1 What a result means depends on your TSH, so pick it above. Free T3 tests are less reliable than free T4 tests.4 The bar isn’t to scale.
In range
Your free T3 is normal. With a normal TSH too, your thyroid is working normally.4 If you haven’t had TSH checked, it’s the better first test.4
Nothing to worry about.
- 1 If you haven’t had TSH checked, add it.4
Normal free T3, but a low TSH: a mildly overactive thyroid
If free T4 is normal too, this is subclinical hyperthyroidism.4 Taking too much thyroid hormone, thyroid nodules and Graves’ disease are the usual causes.4 A low TSH also comes with a serious illness and steroid medicines, and early in pregnancy it’s often normal.3,4
Not urgent, but follow it up, especially if you’re 65 or older or have heart disease or thin bones.
- 1 Check free T4 if it wasn’t done.4
- 2 Repeat TSH, free T4 and T3 within 2 to 6 weeks if you’re 65 or older or have heart disease or thin bones, and within 3 to 6 months otherwise. Guidelines decide on a TSH that stays low, not one result.4
- 3 If you take thyroid hormone, ask whether your dose should come down.5
High, with a low TSH: too much thyroid hormone
You have too much thyroid hormone. If you don’t take thyroid hormone, this means an overactive thyroid. If free T4 is normal, it’s called T3 toxicosis, which can be an early stage of Graves’ disease or a nodule making hormone on its own.4 In about 5% of overactive thyroids, T3 is the only hormone that’s high.2
See a clinician in the next few days. Seek urgent care if your heart is racing or irregular, or you feel very unwell.
- 1 If you take thyroid hormone, especially T3 or desiccated thyroid, your dose may be too high. Ask whether it should come down.5
- 2 Otherwise, ask about a TSH receptor antibody test. A positive result confirms Graves’ disease.4
- 3 Ask whether a beta blocker could calm a racing heart and shakiness while you wait for treatment.4
- 4 If you take biotin, stop it for at least 72 hours, as Labcorp asks, before any repeat test.1
High, without a low TSH: check the test first
A normal TSH all but rules out an overactive thyroid.4 So a high free T3 with a TSH that isn’t low more often points to the test. Biotin supplements and antibodies in your blood that react with the test can both throw off free T3.1
Not urgent on its own.
- 1 Stop biotin for at least 72 hours, as Labcorp asks, then repeat free T3 with TSH and free T4.1
- 2 If you take liothyronine or desiccated thyroid, tell your clinician when you took your last dose.
High: check your TSH
A high free T3 often means an overactive thyroid or too much thyroid hormone, and TSH settles it. If that’s the cause, TSH is low.4
See a clinician in the next week or so, and sooner if your heart is racing.
Low: usually illness or dieting, not your thyroid
Free T3 falls during a serious illness, and with fasting or eating very little, even when the thyroid is healthy.2,3 With a normal TSH, a low free T3 isn’t a sign of an underactive thyroid, and US guidelines don’t use T3 to diagnose one.3
Not a worry on its own.
- 1 Don’t start T3 or thyroid supplements because of this result. US guidelines advise against thyroid hormone without a confirmed diagnosis.3
Low: check your TSH and free T4
Free T3 often falls with an illness, fasting or eating very little.2,3 But it can also be low with an underactive thyroid, so TSH and free T4 show which it is.3
Not urgent.
- 1 Check TSH and free T4.3
Low, with a low TSH
If you’re being treated for an overactive thyroid, this can mean your dose is now too high. TSH can stay low for months after treatment starts.4 Otherwise, it usually comes with a serious illness, steroid treatment or eating very little.2,3 Less often, the pituitary gland isn’t making enough TSH.3
Worth seeing a clinician in the next few weeks, and sooner if you feel unwell.
With a high TSH: check free T4
A high TSH usually means an underactive thyroid, though it can also rise for a while as you recover from an illness.3 Free T3 often stays normal even when the thyroid is clearly underactive, so free T4 and TSH, not free T3, show how far along it is and guide treatment.3
It depends on your TSH. At 10 or more, see a clinician in the next few weeks.3
- 1 Check free T4 and TPO antibodies if they weren’t done.3
- 2 If you already take levothyroxine, your dose may be too low or not being absorbed. Ask whether it should go up.5
What is a normal free T3 level?
Labcorp’s range for adults is 2.0 to 4.4 pg/mL.1 Like free T4, free T3 is read next to TSH, and the same number can mean different things depending on it:4
In range
Your free T3 is normal. With a normal TSH too, your thyroid is working normally.4 If you haven’t had TSH checked, it’s the better first test.4
Nothing to worry about.
- 1 If you haven’t had TSH checked, add it.4
Normal free T3, but a low TSH: a mildly overactive thyroid
If free T4 is normal too, this is subclinical hyperthyroidism.4 Taking too much thyroid hormone, thyroid nodules and Graves’ disease are the usual causes.4 A low TSH also comes with a serious illness and steroid medicines, and early in pregnancy it’s often normal.3,4
Not urgent, but follow it up, especially if you’re 65 or older or have heart disease or thin bones.
- 1 Check free T4 if it wasn’t done.4
- 2 Repeat TSH, free T4 and T3 within 2 to 6 weeks if you’re 65 or older or have heart disease or thin bones, and within 3 to 6 months otherwise. Guidelines decide on a TSH that stays low, not one result.4
- 3 If you take thyroid hormone, ask whether your dose should come down.5
High, with a low TSH: too much thyroid hormone
You have too much thyroid hormone. If you don’t take thyroid hormone, this means an overactive thyroid. If free T4 is normal, it’s called T3 toxicosis, which can be an early stage of Graves’ disease or a nodule making hormone on its own.4 In about 5% of overactive thyroids, T3 is the only hormone that’s high.2
See a clinician in the next few days. Seek urgent care if your heart is racing or irregular, or you feel very unwell.
- 1 If you take thyroid hormone, especially T3 or desiccated thyroid, your dose may be too high. Ask whether it should come down.5
- 2 Otherwise, ask about a TSH receptor antibody test. A positive result confirms Graves’ disease.4
- 3 Ask whether a beta blocker could calm a racing heart and shakiness while you wait for treatment.4
- 4 If you take biotin, stop it for at least 72 hours, as Labcorp asks, before any repeat test.1
High, without a low TSH: check the test first
A normal TSH all but rules out an overactive thyroid.4 So a high free T3 with a TSH that isn’t low more often points to the test. Biotin supplements and antibodies in your blood that react with the test can both throw off free T3.1
Not urgent on its own.
- 1 Stop biotin for at least 72 hours, as Labcorp asks, then repeat free T3 with TSH and free T4.1
- 2 If you take liothyronine or desiccated thyroid, tell your clinician when you took your last dose.
Low: usually illness or dieting, not your thyroid
Free T3 falls during a serious illness, and with fasting or eating very little, even when the thyroid is healthy.2,3 With a normal TSH, a low free T3 isn’t a sign of an underactive thyroid, and US guidelines don’t use T3 to diagnose one.3
Not a worry on its own.
- 1 Don’t start T3 or thyroid supplements because of this result. US guidelines advise against thyroid hormone without a confirmed diagnosis.3
With a high TSH: check free T4
A high TSH usually means an underactive thyroid, though it can also rise for a while as you recover from an illness.3 Free T3 often stays normal even when the thyroid is clearly underactive, so free T4 and TSH, not free T3, show how far along it is and guide treatment.3
It depends on your TSH. At 10 or more, see a clinician in the next few weeks.3
What is free T3?
Think of T4 as the stored form of thyroid hormone and T3 as the working form. Your thyroid makes mostly T4, and your body turns it into T3 where it’s needed.
About 80% of the T3 in your body is made from T4 outside the thyroid.5 About 99.7% of T3 travels bound to proteins; free T3 is the small unbound part your cells can use.1
When is a free T3 test useful?
Mainly when your thyroid may be overactive. TSH falls first. In mild cases, T3 can then rise while free T4 is still normal, so US guidelines check T3 alongside free T4 when an overactive thyroid is suspected.4
It can’t rule out an underactive thyroid. Free T3 often stays normal until late, so US guidelines diagnose one with TSH and free T4 instead.3
Why is free T3 low?
- A serious illness, which slows the conversion of T4 to T33
- Fasting, or eating very little2
- An underactive thyroid, though free T3 often stays normal3
- Amiodarone and high-dose propranolol, two heart medicines that slow conversion4
- Levothyroxine treatment, on which free T3 often sits at the low end of the range5
Does adding T3 to levothyroxine help?
Some people still feel unwell on levothyroxine with a normal TSH, which keeps interest in adding T3 alive. So far, 14 trials haven’t shown a consistent benefit.7
| Treatment | What the evidence shows |
|---|---|
| Levothyroxine alone | The standard treatment. Your body converts it to T3, and TSH guides the dose. |
| Adding T3 (liothyronine) | Raises T3, but trials haven’t shown a consistent benefit over levothyroxine alone. |
| Desiccated thyroid | Dried animal thyroid with both T4 and T3. Not FDA-approved, and US guidelines advise against it. |
From US guidelines, a 2021 consensus statement and the FDA.3,5,6,7
A 2024 trial, LEVOLIO, gave added T3 or a placebo to 141 people whose thyroid had been removed, and found no difference in quality of life after 6 months, whatever their genes.8 A Dutch trial, T3-4-Hypo, is testing added T3 against a placebo in people with autoimmune hypothyroidism and persistent tiredness. It aims to enroll 600 people, with results expected in 2028.9 Until then, US guidelines don’t support adding T3 routinely.3
Who should get a free T3 test?
For an underactive thyroid, or to check a levothyroxine dose, TSH and free T4 are the right tests.3,5
How to get a free T3 test, and what it costs
A free T3 test costs $5.00 through Picardo. The Thyroid & Autoimmunity panel adds 12 more tests from the same visit for $76.50.
Thyroid & Autoimmunity panel
Free T3 means most next to TSH and free T4. The panel adds both, plus total T3, ANA, rheumatoid factor and inflammation tests, a blood count and kidney and liver tests. See what’s in it
Free T3 on its own
Only the number this page is about.
- Results in 2 to 3 days, in the app
- Physician follow-up if you want one
- Free Labcorp draw, or $75 at home
- No other fees. HSA and FSA eligible.
It’s also part of the Postpartum panel, if you want a wider checkup.
Already have a result? Ask Picardo about itQuestions people ask
Why is my free T3 low on levothyroxine?
It’s common. On levothyroxine, free T3 often sits at the low end of the range, or just below it, even when TSH is normal. What that means for how you feel is unknown, and guidelines set the dose by TSH, not T3.5
Is free T3 or total T3 the better test?
For an overactive thyroid, US guidelines say total T3 is often preferred, because free T3 tests are less well validated and less reliable.4 Either one shows whether T3 is high, but total T3 also rises with pregnancy and the pill.2
Should I fast before a free T3 test?
No. But a long fast or a very low-calorie diet lowers T3, so eat normally in the days before.2
Does biotin affect free T3?
It can. High-dose biotin, common in hair, skin and nail supplements, interferes with many thyroid tests. Labcorp asks you to stop it at least 72 hours before the draw.1
Often checked alongside
Test details
- Labcorp test
- Triiodothyronine (T3), Free, 010389
- LOINC
- 3051-0
- Sample
- Blood
- Fasting
- Not needed
- Typical lab range
- 2.0 to 4.4 pg/mL for adults over 191
- Units
- pg/mL in the US. For pmol/L, multiply by 1.536.
Sources
- 1 Labcorp. Triiodothyronine (T3), free, test 010389. Labcorp test menu. Accessed October 28, 2026. www.labcorp.com/tests/010389/triiodothyronine-t3-free
- 2 Labcorp. Triiodothyronine (T3), test 002188. Labcorp test menu. Accessed October 28, 2026. www.labcorp.com/tests/002188/triiodothyronine-t3
- 3 Garber JR, Cobin RH, Gharib H, et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Thyroid. 2012;22(12):1200–1235. doi.org/10.1089/thy.2012.0205
- 4 Ross DS, Burch HB, Cooper DS, et al. 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid. 2016;26(10):1343–1421. doi.org/10.1089/thy.2016.0229
- 5 Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014;24(12):1670–1751. doi.org/10.1089/thy.2014.0028
- 6 U.S. Food and Drug Administration. FDA’s actions to address unapproved thyroid medications. Updated August 6, 2026. www.fda.gov/drugs/enforcement-activities-fda/fdas-actions-address-unapproved-thyroid-medications
- 7 Jonklaas J, Bianco AC, Cappola AR, et al. Evidence-based use of levothyroxine/liothyronine combinations in treating hypothyroidism: a consensus document. Thyroid. 2021;31(2):156–182. doi.org/10.1089/thy.2020.0720
- 8 Brigante G, Santi D, Boselli G, et al. Randomized double-blind placebo-controlled trial on levothyroxine and liothyronine combination therapy in totally thyroidectomized subjects: the LEVOLIO study. Eur J Endocrinol. 2024;190(1):12–22. doi.org/10.1093/ejendo/lvad172
- 9 M. Medici. LT4/LT3 combination therapy versus LT4 monotherapy in patients with autoimmune hypothyroidism (T3-4-Hypo). ClinicalTrials.gov identifier NCT05682482. Updated February 26, 2025. clinicaltrials.gov/study/NCT05682482
Updated 4 Nov 2026.
This is general health information, not medical advice: talk to your clinician about your results and before changing any treatment.