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Free T3 (Free Triiodothyronine)

The unattached, active form of the thyroid hormone T3, which drives how fast your cells use energy. It is usually checked after TSH and free T4.

AI-verified against cited sources. Awaiting expert reviewAlso called FT3, fT3LOINC 3051-0

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What it is

T3 is the more active thyroid hormone. Your thyroid makes a little of it, but most T3 is made in other tissues by converting T4. Like T4, most T3 in the blood is attached to carrier proteins. Free T3 is the small unattached portion that is available to your cells. Clinicians usually order it as a follow-up, for example when TSH is low or when symptoms and other thyroid tests do not seem to match.

Why it matters

Free T3 can help show whether an overactive thyroid is making extra T3 even when free T4 looks normal. A low free T3 is common during illness or fasting and does not always mean a thyroid problem. For people taking thyroid hormone, TSH is the main number used to judge whether treatment is at the right level, so free T3 is not usually needed for routine monitoring.

If it’s low

A free T3 below your lab's range means less active hormone is available than usual. This is often not a thyroid problem. Illness, surgery, fasting and some medicines can lower T3 by slowing the conversion of T4 to T3, and it usually recovers as you do. TSH and free T4 are normally more useful for judging an underactive thyroid.

Common causes

  • A recent illness, hospital stay or surgery
  • Fasting, or a very low-calorie diet
  • Medicines such as amiodarone, propranolol and glucocorticoids
  • An underactive thyroid, usually together with a raised TSH

What can help

  • Ask whether TSH and free T4 should be checked or rechecked once you have recovered from illness
  • Ask whether any of your medicines could be playing a role

If it’s high

A free T3 above your lab's range means more active thyroid hormone is circulating than usual. It is often linked to an overactive thyroid, usually with a low TSH. Some people with an overactive thyroid have a raised T3 with a normal T4. Medicines or supplements that contain T3, or thyroid extract, can also raise it for several hours after a dose, so the timing of your last dose matters.

Common causes

  • Thyroid medicine that contains T3, or thyroid extract supplements, especially when blood is drawn a few hours after a dose
  • Graves' disease or thyroid nodules that make extra hormone
  • Thyroiditis, an inflammation that releases stored hormone for a while
  • Biotin supplements, which can falsely change results on some tests
  • Rarely, antibodies that interfere with the test

What can help

  • Ask whether TSH and free T4 fit with this result
  • Tell your clinician about every supplement you take, including biotin and any sold for thyroid or metabolism support
  • Ask whether to repeat the test

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

The usual range depends on age. Choose an age to see the one that applies.

Children and teens 3 ranges

See Free T3 by age and sex, with the U.S. survey numbers where they exist.

If this applies to you

Free T3 means a little more for some people. Open the one that fits you.

Thyroid condition

Free T3 is mostly used for an overactive thyroid. NICE suggests checking TSH, free T4 and free T3 every 6 weeks on antithyroid medicine until TSH is back in range. For people taking levothyroxine, the ATA calls TSH the most reliable guide to the dose.

Blood tests for a thyroid condition

Endurance athlete

Free T3 is an active thyroid hormone, and under-fuelling can lower it. The IOC counts a T3 in the lowest quarter of the lab's range, or below it, as a sign of REDs.

Blood tests for endurance athletes

What can affect your result

  • Illness, surgery, fasting and calorie restriction
  • Medicines such as amiodarone, propranolol and glucocorticoids
  • Thyroid medicine or extracts that contain T3, and the time since the last dose
  • Age: children have different ranges than adults
  • Biotin supplements
  • Test method and lab
  • High-dose biotin can interfere with some immunoassays. Labcorp asks patients to stop biotin at least 72 hours before the draw; check with your clinician or lab first.
  • Antibodies to thyroid hormones can interfere with free hormone immunoassays, which can give a result that does not fit the rest of your thyroid tests (Labcorp).
  • Serious illness, surgery, fasting and very low-calorie diets can lower free T3 because less T4 is converted to T3.
  • Medicines such as amiodarone, propranolol and glucocorticoids can lower the conversion of T4 to T3.
  • Thyroid medicine that contains T3, or thyroid extract, can raise free T3 for several hours after a dose.
  • Labcorp says not to use oxalate, EDTA or citrate plasma for this test.

Free T3 can drop within days of an acute illness or severe calorie restriction and recover as you do. After a change in thyroid treatment, it takes weeks to settle.

When to talk to a clinician

Talk with a clinician about any free T3 outside your lab's range, especially if you have symptoms of an overactive thyroid such as a racing heart, tremor, heat intolerance, weight loss or poor sleep. No single universal cutoff marks an urgent result. If you have chest pain, a very fast or irregular heartbeat, fainting or confusion, call 911 or your local emergency number.

Questions you could ask

  • Is a free T3 test useful for me, or are TSH and free T4 enough?
  • How should I time my thyroid medicine around a blood draw?
  • Does this result fit with my TSH and free T4?
  • Could a supplement or the timing of a medicine dose explain it?
  • Should we repeat the test?
  • Could a recent illness, diet or medicine explain this?

Questions people ask

Do I need a free T3 test if my TSH is normal?

Usually not. TSH, often with free T4, is the first step. Free T3 is mainly used as a follow-up, for example when TSH is low or when symptoms do not match other results. Your clinician can tell you whether it adds anything for you.

Why is my free T3 low when my other thyroid tests look fine?

Illness, fasting, calorie restriction and some medicines can lower free T3 without a real thyroid problem. It often returns to normal on its own. Ask your clinician whether to repeat the test once you have recovered.

How do I convert pg/mL to pmol/L?

Multiply pg/mL by 1.536, so 3.0 pg/mL is about 4.6 pmol/L. Some labs print ng/dL instead, and 1 ng/dL equals 10 pg/mL.

Can biotin or thyroid supplements change my free T3?

Yes. Biotin can interfere with some tests, and supplements that contain T3 or thyroid extract can raise free T3 for hours after a dose. Tell your clinician about everything you take, and ask about timing before a blood draw.

Show the science and sources

Units

  • 1 pg/mL = 1 pg/mL
  • 1 pg/mL = 1.536 pmol/L
  • 1 pg/mL = 0.1 ng/dL
  • 1 pg/mL = 1 ng/L

Open the Free T3 unit converter

Is a change real?

Free T3 naturally varies about 5.1% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 17% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Biological variation estimates of thyroid related measurands - meta-analysis of BIVAC compliant studies (Fernandez-Calle P et al., EFLM WG-BV).

For clinicians and researchers

Most circulating T3 comes from peripheral deiodination of T4 by type 1 and type 2 deiodinases, with a smaller share secreted by the thyroid. Only a tiny fraction of T3 is unbound, and free T3 is generally measured by automated immunoassay. Labcorp and Mayo Clinic Laboratories both list a Roche cobas electrochemiluminescence immunoassay with a 2.0 to 4.4 pg/mL adult interval. Immunoassays for free hormones are vulnerable to binding-protein changes, anti-T3 autoantibodies and biotin. Mayo positions free T3 as a second- or third-level test: to confirm hyperthyroidism alongside T4, sensitive TSH and total T3, and to evaluate clinically euthyroid patients with altered binding proteins. In its thyroid monitoring cascade, Mayo adds a total T3 (not free T3) only if TSH is below 0.1 mIU/L and free T4 is normal. Non-thyroidal illness lowers T3 through reduced conversion and increased reverse T3, and the low T3 usually resolves with recovery. The ATA 2014 guideline names TSH as the most reliable marker of adequate levothyroxine replacement. Liothyronine monotherapy produces peaks and troughs in T3, and older studies cited by the ATA 2014 guideline found raised T3 2 to 5 hours after thyroid extract, so a result drawn soon after a dose can look high.

LOINC 3051-0 (primary), 14928-6, 29239-1. Specimen: serum-or-plasma.

Sources

  1. Triiodothyronine (T3), Free (test 010389), Labcorp
  2. T3 (Triiodothyronine), Free, Serum (test T3FR), Mayo Clinic Laboratories (Extended Catalog, testcatalog.org)
  3. Thyroid Function Cascade, Monitor (without Thyroid Peroxidase Antibody), Serum (test THCSM), Mayo Clinic Laboratories (Extended Catalog, testcatalog.org)
  4. Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement, Thyroid (2014)
  5. Biotin Interference with Routine Clinical Immunoassays: Understand the Causes and Mitigate the Risks, Endocrine Practice (2017)
  6. Biotin Treatment Mimicking Graves' Disease, New England Journal of Medicine (2016)
  7. Liothyronine (PubChem compound 5920), PubChem, National Library of Medicine

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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