The short answer
Free T4 is the main hormone your thyroid makes, measured as the part your body can use. It’s read with TSH: together, the two show whether your thyroid is underactive, overactive or working normally. Labcorp’s adult range is 0.82 to 1.77 ng/dL.1,2
- 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. Biotin and a levothyroxine dose just before the draw can push free T4 up.2,3
- If yours is low
- With a high TSH, your thyroid is underactive, which is very treatable. Without a high TSH, the cause is usually elsewhere, such as the pituitary gland, an illness or a medicine.2
Check your result
Type the number from your report. It stays on this page.
- Low Under 0.82
- In range 0.82 to 1.77
- High 1.78 or more
- Low, TSH not high Under 0.82
- Normal, TSH low 0.82 to 1.77
- Overactive 1.78 or more
- Low, TSH not high Under 0.82
- In range 0.82 to 1.77
- High, TSH not low 1.78 or more
- Underactive Under 0.82
- Normal, TSH high 0.82 to 1.77
- High, TSH not low 1.78 or more
- Low on treatment Under 0.82
- In range on treatment 0.82 to 1.77
- High on treatment 1.78 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 T4 cutoffs are Labcorp’s adult range.1 What a result means depends on your TSH, so pick it above. Ranges vary between labs, and free T4 tests can read lower in pregnancy.7 The bar isn’t to scale.
In range
Your free T4 is normal. With a normal TSH too, your thyroid is working normally.2 If you haven’t had TSH checked, it’s worth adding. TSH reacts to small changes in thyroid hormone, so it can be abnormal while free T4 is still in range.2
Nothing to worry about.
- 1 If you haven’t had TSH checked, add it.
Normal free T4, but a high TSH: a mildly underactive thyroid
This pattern is called subclinical hypothyroidism. Your thyroid is still making enough hormone, but your pituitary gland is pushing harder to keep it that way.4 A single high TSH can be temporary, so guidelines advise repeating the tests after 2 to 3 months before deciding anything.4
Not urgent. Most people with this pattern don’t need treatment straight away.
- 1 Repeat TSH and free T4 in 2 to 3 months, and add TPO antibodies.4
- 2 If your TSH stays at 10 or higher, levothyroxine is usually considered. Below 10, it depends on your age, your symptoms and whether you’re trying to get pregnant.2,4
- 3 If you’re pregnant, don’t wait 2 to 3 months. Repeat within a few weeks, and in the first trimester ask whether levothyroxine makes sense.7
Low, with a high TSH: an underactive thyroid
This is overt hypothyroidism. Your thyroid isn’t making enough hormone, and your pituitary gland is calling for more.2 The most common cause is autoimmune thyroid disease, often called Hashimoto’s.2 A daily levothyroxine tablet replaces the missing hormone.2
Very treatable. See a clinician in the next week or two to start treatment.
- 1 Ask about starting levothyroxine. If you already take it, this means your dose is too low or isn’t being absorbed.2,9
- 2 Check TPO antibodies to see whether Hashimoto’s is the cause.2
- 3 Once you start, expect a TSH check 4 to 6 weeks after each dose change.9
Low, without a high TSH: look beyond the thyroid
If your thyroid were failing, your TSH would usually be high. When it isn’t, the cause is often elsewhere. The pituitary gland may not be making enough TSH, called central hypothyroidism.2 A serious illness, recent treatment for an overactive thyroid and some medicines can cause the same pattern.2 Resmetirom, a liver drug approved in 2024, lowers free T4 by about 13% to 17% with little change in TSH.6
Worth seeing a clinician in the next few weeks, and sooner if you feel unwell.
- 1 If you’re being treated for an overactive thyroid, tell the clinician treating you soon. Your thyroid may now be underactive even though TSH hasn’t caught up, and your dose may need lowering or levothyroxine may be needed.2,3
- 2 Otherwise, tell your clinician about any recent illness and every medicine you take, and repeat TSH and free T4 to confirm the result.
- 3 In the second half of pregnancy, standard free T4 tests read lower, so a result just under the range may be normal.7
Normal free T4, but a low TSH: a mildly overactive thyroid
With T3 also normal, this is called subclinical hyperthyroidism.3 If T3 is high, it’s an overactive thyroid that shows up in T3 first, as it can early in Graves’ disease.3 Taking too much thyroid hormone, thyroid nodules and Graves’ disease are the usual causes.3 A low TSH also comes with a serious illness and steroid medicines, and early in pregnancy it’s often normal.2,3
Not urgent, but follow it up, especially if you’re 65 or older or have heart disease or thin bones.
- 1 Check T3 if it hasn’t been done.3
- 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 a single result.3
- 3 If your TSH stays under 0.1 and you’re 65 or older, have heart disease or its risk factors, thin bones or symptoms, or are past menopause and not on estrogen or a bone medicine, treatment is recommended.3
- 4 If you take levothyroxine, ask whether your dose should come down.9
High, with a low TSH: too much thyroid hormone
You have too much thyroid hormone in your blood. If you don’t take thyroid hormone, this is overt hyperthyroidism, an overactive thyroid.3 Graves’ disease, an autoimmune condition, is the most common cause; thyroid nodules, inflammation of the thyroid and taking too much thyroid hormone are others.3 Left untreated, it can lead to an irregular heartbeat and bone loss.3
See a clinician in the next few days. Seek urgent care if your heart is racing or irregular, or you feel very unwell.
- 1 Ask about a TSH receptor antibody test. A positive result confirms Graves’ disease.3
- 2 Ask whether a beta blocker could calm a racing heart and shakiness while you wait for treatment.3
- 3 If you take thyroid hormone, your dose may be too high. Ask whether it should come down.9
- 4 If you take biotin, stop it for at least 72 hours, as Labcorp asks, then repeat the test. Biotin can mimic this pattern.1,3
High, without a low TSH: check the test first
A normal TSH all but rules out an overactive thyroid.3 So a high free T4 with a TSH that isn’t low usually comes from outside the thyroid. Biotin supplements, a levothyroxine dose in the hours before the draw, heparin and some heart medicines can all push free T4 up.1,2,3 If you take levothyroxine, a free T4 a little above the range with a normal TSH is common.9 Rarely, it’s an inherited condition or a pituitary tumor, and those are diagnosed only once a test problem has been ruled out.5
Not urgent on its own.
- 1 Stop biotin for at least 72 hours, as Labcorp asks, then repeat TSH and free T4.1
- 2 If you take levothyroxine, take it after the blood draw on test day.2
- 3 If it’s still high, ask whether the lab can check it another way.5
High: check your TSH
A high free T4 often means an overactive thyroid, and TSH settles it. If your thyroid is the cause, TSH is low.3 Biotin and a levothyroxine dose just before the draw can also push free T4 up.2,3
See a clinician in the next week or so, and sooner if your heart is racing.
Low: check your TSH
A low free T4 means you may not be making enough thyroid hormone. TSH shows where the problem is. It’s high when the thyroid itself is failing, the most common cause, and normal or low when the pituitary gland is.2
Worth seeing a clinician in the next week or two.
- 1 Check TSH.2
Low on thyroid hormone: check your TSH
On levothyroxine, a low free T4 usually means your dose is too low or isn’t being absorbed, and a high TSH would confirm it.9 If your thyroid problem is in the pituitary gland, free T4 rather than TSH guides your dose.2
Not urgent, but worth seeing a clinician in the next few weeks.
- 1 Check TSH if it wasn’t done.9
- 2 Take levothyroxine the same way every day, 60 minutes before breakfast or at bedtime, and apart from calcium and iron.9
In range on thyroid hormone
Your free T4 is normal. On levothyroxine, TSH is the number that guides your dose, unless your thyroid problem is in the pituitary gland, when free T4 does.2,9
Nothing to worry about.
- 1 Once your dose is settled, check TSH about once a year.9
A little high on thyroid hormone: often expected
On levothyroxine, free T4 often runs higher than in people not taking it, and it rises for several hours after each dose.2,9 If your TSH is in range, your dose is usually fine. If TSH is low, the dose may be too high.9
Not a worry on its own.
What is a normal free T4 level?
Labcorp’s range for adults is 0.82 to 1.77 ng/dL.1 But free T4 is rarely read on its own: the same number means different things depending on your TSH.2,3 Here’s what each combination means:
In range
Your free T4 is normal. With a normal TSH too, your thyroid is working normally.2 If you haven’t had TSH checked, it’s worth adding. TSH reacts to small changes in thyroid hormone, so it can be abnormal while free T4 is still in range.2
Nothing to worry about.
- 1 If you haven’t had TSH checked, add it.
Normal free T4, but a high TSH: a mildly underactive thyroid
This pattern is called subclinical hypothyroidism. Your thyroid is still making enough hormone, but your pituitary gland is pushing harder to keep it that way.4 A single high TSH can be temporary, so guidelines advise repeating the tests after 2 to 3 months before deciding anything.4
Not urgent. Most people with this pattern don’t need treatment straight away.
- 1 Repeat TSH and free T4 in 2 to 3 months, and add TPO antibodies.4
- 2 If your TSH stays at 10 or higher, levothyroxine is usually considered. Below 10, it depends on your age, your symptoms and whether you’re trying to get pregnant.2,4
- 3 If you’re pregnant, don’t wait 2 to 3 months. Repeat within a few weeks, and in the first trimester ask whether levothyroxine makes sense.7
Low, with a high TSH: an underactive thyroid
This is overt hypothyroidism. Your thyroid isn’t making enough hormone, and your pituitary gland is calling for more.2 The most common cause is autoimmune thyroid disease, often called Hashimoto’s.2 A daily levothyroxine tablet replaces the missing hormone.2
Very treatable. See a clinician in the next week or two to start treatment.
Low, without a high TSH: look beyond the thyroid
If your thyroid were failing, your TSH would usually be high. When it isn’t, the cause is often elsewhere. The pituitary gland may not be making enough TSH, called central hypothyroidism.2 A serious illness, recent treatment for an overactive thyroid and some medicines can cause the same pattern.2 Resmetirom, a liver drug approved in 2024, lowers free T4 by about 13% to 17% with little change in TSH.6
Worth seeing a clinician in the next few weeks, and sooner if you feel unwell.
- 1 If you’re being treated for an overactive thyroid, tell the clinician treating you soon. Your thyroid may now be underactive even though TSH hasn’t caught up, and your dose may need lowering or levothyroxine may be needed.2,3
- 2 Otherwise, tell your clinician about any recent illness and every medicine you take, and repeat TSH and free T4 to confirm the result.
- 3 In the second half of pregnancy, standard free T4 tests read lower, so a result just under the range may be normal.7
Normal free T4, but a low TSH: a mildly overactive thyroid
With T3 also normal, this is called subclinical hyperthyroidism.3 If T3 is high, it’s an overactive thyroid that shows up in T3 first, as it can early in Graves’ disease.3 Taking too much thyroid hormone, thyroid nodules and Graves’ disease are the usual causes.3 A low TSH also comes with a serious illness and steroid medicines, and early in pregnancy it’s often normal.2,3
Not urgent, but follow it up, especially if you’re 65 or older or have heart disease or thin bones.
- 1 Check T3 if it hasn’t been done.3
- 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 a single result.3
- 3 If your TSH stays under 0.1 and you’re 65 or older, have heart disease or its risk factors, thin bones or symptoms, or are past menopause and not on estrogen or a bone medicine, treatment is recommended.3
- 4 If you take levothyroxine, ask whether your dose should come down.9
High, with a low TSH: too much thyroid hormone
You have too much thyroid hormone in your blood. If you don’t take thyroid hormone, this is overt hyperthyroidism, an overactive thyroid.3 Graves’ disease, an autoimmune condition, is the most common cause; thyroid nodules, inflammation of the thyroid and taking too much thyroid hormone are others.3 Left untreated, it can lead to an irregular heartbeat and bone loss.3
See a clinician in the next few days. Seek urgent care if your heart is racing or irregular, or you feel very unwell.
- 1 Ask about a TSH receptor antibody test. A positive result confirms Graves’ disease.3
- 2 Ask whether a beta blocker could calm a racing heart and shakiness while you wait for treatment.3
- 3 If you take thyroid hormone, your dose may be too high. Ask whether it should come down.9
- 4 If you take biotin, stop it for at least 72 hours, as Labcorp asks, then repeat the test. Biotin can mimic this pattern.1,3
High, without a low TSH: check the test first
A normal TSH all but rules out an overactive thyroid.3 So a high free T4 with a TSH that isn’t low usually comes from outside the thyroid. Biotin supplements, a levothyroxine dose in the hours before the draw, heparin and some heart medicines can all push free T4 up.1,2,3 If you take levothyroxine, a free T4 a little above the range with a normal TSH is common.9 Rarely, it’s an inherited condition or a pituitary tumor, and those are diagnosed only once a test problem has been ruled out.5
Not urgent on its own.
What is free T4?
Think of TSH as the order and free T4 as the delivery. When deliveries run short, your pituitary gland sends bigger orders. Reading the two together shows whether a problem starts in your thyroid or in the gland placing the orders.
T4, or thyroxine, is the main hormone your thyroid makes. About 99.97% of it travels bound to proteins in your blood. The rest is free T4, the part your body can use, and it’s what this test measures.2
Your pituitary gland watches free T4 and adjusts TSH to keep it steady. TSH reacts to very small changes, which is why it usually turns abnormal first.2,3
How to read free T4 with your TSH
| Your TSH | Free T4 low | Free T4 in range | Free T4 high |
|---|---|---|---|
| TSH high | Underactive thyroid | Mildly underactive (subclinical) | Unusual. Check the test first |
| TSH in range | Pituitary, illness or a medicine | Normal | Unusual. Check the test first |
| TSH low | Pituitary, illness or recent treatment | Mildly overactive (subclinical) | Too much thyroid hormone |
The pattern matters more than either number alone. A high TSH with a normal free T4 is a mild change that can be temporary, while a high free T4 with a normal TSH is more often a test problem than a thyroid problem.3,4,5
What can throw off a free T4 result?
- Biotin supplements, which can make free T4 read falsely high3,5
- A levothyroxine dose in the hours before the draw2
- Heparin, a blood thinner, and amiodarone and propranolol, two heart medicines, which can raise it1
- Carbamazepine, a seizure medicine, and resmetirom, a liver drug, which lower it1,6
- Pregnancy, when standard free T4 tests can read lower7
- A serious illness2
- Rarely, antibodies in your blood that react with the test5
How is a low or high free T4 treated?
It depends on the cause, which your TSH and other tests point to.
- Levothyroxine, for an underactive thyroid
A daily tablet that replaces the T4 your thyroid isn’t making. The dose is set by TSH, rechecked 4 to 6 weeks after each change.9
- How much it helps
- A lot
- How sure we are
- Very sure
- Antithyroid medicine, for an overactive thyroid
Methimazole slows hormone production. It’s one of three standard treatments for Graves’ disease, and the one many doctors outside the US prefer to start with.3
- How much it helps
- A lot
- How sure we are
- Very sure
- Radioactive iodine
A single dose, taken by mouth. For Graves’ disease the aim is to leave the thyroid underactive, so most people then take levothyroxine.3
- How much it helps
- A lot
- How sure we are
- Very sure
- Surgery
Removing the thyroid cures an overactive thyroid, and is favored for a large goiter or a suspicious nodule. Levothyroxine replaces the hormone afterwards.3
- How much it helps
- A lot
- How sure we are
- Very sure
- Beta blockers
Calm a racing heart, tremor and anxiety while other treatment takes effect. They don’t treat the thyroid itself, so free T4 stays high until other treatment works.3
- How much it helps
- A little
- How sure we are
- Very sure
Drugs that clear the antibodies behind Graves’ disease are now being tested. Efgartigimod is in phase 3 trials in people whose Graves’ disease isn’t controlled by antithyroid medicine, with the main results due in 2028.8
Who should get a free T4 test?
Free T4 is most useful as a second test. It’s worth checking if:
- Your TSH is high or low.2,3
- You have symptoms of an overactive thyroid, such as weight loss, a racing heart, tremor or anxiety.3
- You have a pituitary condition, where TSH can mislead.2
- You’ve been treated for an overactive thyroid in the last few months.2
How to get a free T4 test, and what it costs
A free T4 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 T4 means most next to TSH. The panel adds TSH, free and total T3, ANA, rheumatoid factor and inflammation tests, plus a blood count and kidney and liver tests. See what’s in it
Free T4 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
Should I take levothyroxine before a free T4 test?
Take it after the blood draw on test day. Free T4 rises by up to about 20% after a dose and stays raised for several hours, which can make a normal result look high.2
Does biotin affect free T4?
Yes. High doses, common in hair, skin and nail supplements, can make free T4 read falsely high and TSH falsely low, a pattern that looks like an overactive thyroid.3,5 Labcorp asks you to stop biotin at least 72 hours before the draw.1
What’s the difference between free T4 and total T4?
About 99.97% of the T4 in your blood is bound to proteins. Free T4 is the small unbound part your body can use. Total T4 rises and falls with those proteins, which go up with pregnancy and estrogen, so free T4 is the usual test.2 In pregnancy, labs ideally use free T4 ranges set for each trimester.7
Is free T4 different in pregnancy?
Yes. Pregnancy changes the proteins that carry T4, and standard free T4 tests can read falsely low, especially in the second half. The 2026 ATA guideline prefers ranges a lab has set for each trimester.7
Often checked alongside
Test details
Sources
- 1 Labcorp. Thyroxine (T4), free, direct, test 001974. Labcorp test menu. Accessed October 28, 2026. www.labcorp.com/tests/001974/thyroxine-t4-free-direct
- 2 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
- 3 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
- 4 Pearce SHS, Brabant G, Duntas LH, et al. 2013 ETA guideline: management of subclinical hypothyroidism. Eur Thyroid J. 2013;2(4):215–228. doi.org/10.1159/000356507
- 5 Campi I, Feldt-Rasmussen U, Gruson D, et al. 2026 ETA guideline on interference in immunoassay measurements used in assessment of thyroid function. Eur Thyroid J. 2026;15(4):ETJ260011. doi.org/10.1530/ETJ-26-0011
- 6 Madrigal Pharmaceuticals. Rezdiffra (resmetirom) tablets, prescribing information. Revised July 2026. DailyMed. dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=e67ea09f-a840-439c-86c8-f98585f978b2
- 7 Korevaar TIM, Leung AM, Alexander EK, et al. American Thyroid Association 2026 guidelines for thyroid disease in preconception, pregnancy, and postpartum. Thyroid. 2026;36(5):481–544. doi.org/10.1177/10507256261445624
- 8 argenx. A study to assess efficacy and safety of efgartigimod PH20 SC PFS in adult participants with Graves’ disease (VitaliThy). ClinicalTrials.gov identifier NCT07570316. Updated November 3, 2026. clinicaltrials.gov/study/NCT07570316
- 9 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
Updated 4 Nov 2026.
This is general health information, not medical advice: talk to your clinician about your results and before changing any treatment.