Free T4 (Free Thyroxine)
The unattached, ready-to-use part of thyroxine (T4), the main hormone your thyroid makes. It shows how much thyroid hormone is available to your body.
AI-verified against cited sources. Awaiting expert reviewAlso called FT4, fT4LOINC 3024-7
Check your result
Type your result to see where it falls.
Nothing you type here leaves your browser.
What it is
Your thyroid mostly makes thyroxine, called T4. Most T4 in the blood is attached to carrier proteins and is not available to your cells. Free T4 is the small unattached portion that can reach your tissues, where some of it is turned into the more active hormone, T3. Free T4 is usually read together with TSH. TSH shows what the pituitary is asking for, and free T4 shows how much hormone is actually there.
Why it matters
Free T4 helps show whether a TSH outside the range reflects a real change in thyroid hormone supply. Clinicians call a raised TSH with a free T4 in range subclinical hypothyroidism, and a raised TSH with a low free T4 overt hypothyroidism. A low TSH with a high free T4 is linked to an overactive thyroid. Free T4 is also used to follow thyroid treatment and to look at pituitary function.
If it’s low
A free T4 below your lab's range means less thyroid hormone is available than usual. With a raised TSH, this pattern is linked to an underactive thyroid. With a low or normal TSH it can reflect serious illness or, rarely, a pituitary problem, so the whole set of results matters.
Common causes
- An underactive thyroid, often from Hashimoto's thyroiditis
- Thyroid hormone medicine at too low a dose, missed doses, or doses taken together with food, calcium or iron, which can reduce absorption
- Serious illness or a hospital stay
- Pregnancy, when ranges change by trimester
- Past thyroid surgery or radioactive iodine treatment
- Some medicines, such as carbamazepine, on some tests
- Rarely, a pituitary problem
What can help
- Ask whether TSH and TPO antibodies should be checked or repeated
- If you take thyroid medicine, ask your clinician how and when to take it, including timing with food, calcium and iron
- Ask whether to repeat the test after an illness has passed
If it’s high
A free T4 above your lab's range means more thyroid hormone is available than usual. It is often linked to an overactive thyroid, usually with a low TSH. It can also come from thyroid hormone medicine taken just before the blood draw, or from a lab effect such as biotin or heparin. When TSH is normal, those other explanations are looked at first, and the test is often repeated.
Common causes
- Thyroid hormone medicine, especially blood drawn a few hours after a dose, or a dose higher than needed
- Biotin supplements, which can falsely change results on some tests
- Heparin given before the draw
- Early pregnancy, when pregnancy hormones can stimulate the thyroid for a while
- Some medicines and contrast dyes, including amiodarone, propranolol and iodine-containing X-ray dye
- Graves' disease or thyroid nodules that make extra hormone
- Thyroiditis, an inflammation that releases stored hormone for a while
What can help
- Ask whether the result fits with your TSH and how you feel
- Tell your clinician about biotin, supplements sold for thyroid support, and when you took your last dose of any thyroid medicine
- Ask whether to repeat the test, or to use a different method
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 sex and age. Choose women or men to see the one that applies.
- 20+0.82–1.77 ng/dL
Sources: Labcorp, ARUP Laboratories, Quest Diagnostics Nichols Institute (listed in the Mayo Clinic Laboratories Extended Catalog, testcatalog.org), Thyroid
- Women, pregnancy t10.9–1.4 ng/dL
Sources: ARUP Laboratories, Quest Diagnostics Nichols Institute (listed in the Mayo Clinic Laboratories Extended Catalog, testcatalog.org)
- Women, pregnancy t20.7–1.3 ng/dL
Sources: ARUP Laboratories, Quest Diagnostics Nichols Institute (listed in the Mayo Clinic Laboratories Extended Catalog, testcatalog.org)
See Free T4 by age and sex, with the U.S. survey numbers where they exist.
If this applies to you
Free T4 means a little more for some people. Open the one that fits you.
Thyroid condition
Free T4 is the main thyroid hormone itself, and it is read together with TSH. NICE suggests adding it when symptoms continue after starting levothyroxine. In Graves' disease it helps show early progress, because TSH can stay low for months.
Pregnant or postpartum
Free T4 is read together with TSH. The ATA's 2026 guideline prefers the lab's own trimester-specific free T4 ranges, notes that free T4 results differ a lot between test methods, and says TSH usually reflects thyroid function best in pregnancy.
What can affect your result
- Test method: equilibrium dialysis with mass spectrometry and immunoassays differ
- Age: children have different ranges than adults
- Pregnancy: ranges differ by trimester and by method
- Medicines, including thyroid hormone, amiodarone, propranolol and heparin
- Timing of a levothyroxine dose before the draw
- Biotin supplements
- Serious illness
- Binding-protein changes from pregnancy or estrogen
- High-dose biotin can falsely change results on some immunoassays, and has caused patterns that mimic an overactive thyroid. Labcorp asks patients to stop biotin at least 72 hours before the draw; check with your clinician or lab first.
- Levothyroxine taken a few hours before the draw: free T4 peaks about 15% above its usual level around 4 hours after a dose (ATA 2014).
- Heparin given before the draw can raise free T4 on some assays, as can iodine-containing X-ray contrast dye, propranolol and amiodarone (Labcorp).
- Carbamazepine, a seizure medicine, can lower results (Labcorp).
- Pregnancy, estrogen-containing medicines, serious illness and rare inherited binding-protein changes can shift immunoassay results without a true change in thyroid hormone.
- Rarely, antibodies to thyroid hormones can interfere with free hormone immunoassays (Labcorp).
- Methods differ: equilibrium dialysis with mass spectrometry and immunoassays can give different numbers and different ranges.
Time of day does not change the free T4 interval in a very large study, but if you take levothyroxine, a dose taken a few hours before the draw can raise the result. Ask your clinician how to time your dose around the blood draw. Several weeks. T4 has a half-life of about 7 days, so levels take roughly 5 to 6 weeks to settle after a change in thyroid hormone treatment.
When to talk to a clinician
Talk with a clinician about any free T4 outside your lab's range, especially if you have symptoms of an underactive thyroid (unusual tiredness, feeling cold, constipation) or an overactive one (racing heart, tremor, heat intolerance, weight loss). No single universal cutoff marks an urgent result. If you are pregnant or planning to be, tell your prenatal clinician about any free T4 outside your lab's range. 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
- Which test method does my lab use for free T4?
- If I take thyroid medicine, how should I time my dose around a blood draw?
- If I am pregnant or planning to be, which free T4 range should we use?
- Does this result fit with my TSH and how I feel?
- Could a supplement, a medicine or the timing of my thyroid medicine explain it?
- Should we repeat the test or use a different method?
Questions people ask
Why is my free T4 range different from someone else's?
Free T4 ranges depend on the test method and the lab. An immunoassay and an equilibrium dialysis test can give different numbers for the same blood. Compare your result with the range printed on your own report.
How do I convert ng/dL to pmol/L?
Multiply ng/dL by 12.87, so 1.2 ng/dL is about 15.4 pmol/L. To go the other way, multiply pmol/L by 0.0777.
Should I take my thyroid medicine before a free T4 blood test?
A dose taken a few hours before the draw can push free T4 up by about 15% for a while. The most useful thing is to be consistent from one test to the next, so ask your clinician how to time your dose around the blood draw.
Can biotin change my free T4?
It can on some tests. High-dose biotin has caused free T4 to read falsely high, which can look like an overactive thyroid. Labcorp asks patients to stop biotin at least 72 hours before the draw. Check with your clinician or lab about your own situation first.
Is free T4 enough on its own?
Usually not. It is read with TSH, because the two together show how the pituitary and the thyroid are working. Sometimes free T3 or thyroid antibodies are added.
Show the science and sources
Units
- 1 ng/dL = 1 ng/dL
- 1 ng/dL = 12.87 pmol/L
- 1 ng/dL = 10 ng/L
Open the Free T4 unit converter
Is a change real?
Free T4 naturally varies about 4.8% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 18% 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
Free T4 is a tiny fraction of total T4 and is the fraction available to cells, but most labs estimate it by automated immunoassay rather than measuring it directly. Labcorp and ARUP list a Roche cobas electrochemiluminescence immunoassay; Quest also offers equilibrium dialysis with chromatography and mass spectrometry, the reference-style approach that is less affected by binding proteins. Immunoassays are vulnerable to albumin and TBG changes (pregnancy, estrogen, severe illness), inherited binding-protein variants, anti-T4 autoantibodies, biotin, and in-vitro lipolysis after heparin that displaces T4 from binding proteins and raises the result. Reference intervals are method-dependent and standardization across assays is incomplete. In a very large single-method study, sex, age, hour of day and time of year did not alter the free T4 interval (Ehrenkranz 2015). On levothyroxine, free T4 shows a transient peak of about 15% around 4 hours after the dose, and patients treated to a normal TSH often run a higher free T4 (ATA 2014). T4's half-life is about 7 days, and steady state is reached about 6 weeks after starting levothyroxine (ATA 2014). Free T4 and TSH have a roughly log-linear relationship in a healthy pituitary, which is why a free T4 inside its range can still sit with a raised TSH. In central hypothyroidism the TSH is not a reliable guide, and free T4 is the primary test. Published pregnancy intervals are strongly method-dependent: ARUP (immunoassay) prints 0.9 to 1.4 ng/dL in the first trimester and 0.7 to 1.3 in the second, while Quest (equilibrium dialysis, LC-MS/MS) prints 0.9 to 2.0, 0.8 to 1.5 and 0.8 to 1.7 by trimester. The ATA 2017 pregnancy guideline recommends population- and trimester-specific ranges for TSH.
LOINC 3024-7 (primary), 14920-3, 6892-4, 70217-5. Specimen: serum-or-plasma.
Sources
- Thyroxine (T4), Free, Direct (test 001974), Labcorp
- Thyroxine, Free (Free T4) (test 0070138), ARUP Laboratories
- T4, Free, Direct Dialysis (test FFT4F), Quest Diagnostics Nichols Institute (listed in the Mayo Clinic Laboratories Extended Catalog, testcatalog.org)
- Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement, Thyroid (2014)
- 2013 ETA Guideline: Management of Subclinical Hypothyroidism, European Thyroid Journal (European Thyroid Association) (2013)
- 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum, Thyroid (2017)
- Circadian and Circannual Rhythms in Thyroid Hormones: Determining the TSH and Free T4 Reference Intervals Based Upon Time of Day, Age, and Sex, Thyroid (2015)
- Biotin Interference with Routine Clinical Immunoassays: Understand the Causes and Mitigate the Risks, Endocrine Practice (2017)
- Biotin Treatment Mimicking Graves' Disease, New England Journal of Medicine (2016)
- Thyroid function tests (MedlinePlus Medical Encyclopedia), MedlinePlus, National Library of Medicine
- Thyroxine (PubChem compound 5819), 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.