Free Thyroxine (FT4)
Free T4 is the small, usable share of thyroid hormone your tissues can actually reach. It's the number that tells you whether a borderline TSH is hiding a real problem.
Part of the Thyroid Panel — see all 9 values together, including Anti-Thyroglobulin Antibodies, Anti-Thyroid Peroxidase Antibodies, Thyroglobulin.
Two numbers on the same thyroid report can seem to disagree. The TSH comes back inside the reference range, the visit ends with "your thyroid looks fine," and yet the Free T4 printed right beside it sits at the very bottom of its own range. Which line is telling the truth?
Free thyroxine, almost always labeled Free T4 or FT4, is the test that settles that kind of argument. Where TSH is the pituitary's signal about how hard the thyroid should be working, Free T4 measures what the gland actually delivered: the usable hormone circulating in your blood at the moment of the draw.
The word doing the work is "free." Almost all the thyroxine in your blood travels locked to carrier proteins, where it sits inert as a backup supply. Picture a workforce mostly tied up in long-term contracts: those people are on the payroll but can't take on anything new today. Only the small share that's off contract, the free fraction, can clock in and do the job your tissues need. Free T4 counts that available share. Total T4 counts everyone on the books, contracted and free together, which is why the older measurement can mislead and why modern panels report the free number instead. The proteins doing the binding are led by thyroid-binding globulin, and when their level shifts, total thyroxine shifts with them while the free hormone often doesn't budge.
One practical note before the numbers. Free T4 is reported in ng/dL by most US labs and in pmol/L across most of the rest of the world, and the two scales are not interchangeable. Read your value against the range printed beside it on the same report, never against a number you remember from somewhere else.
What the numbers usually mean
ng/dLPoints toward an underactive thyroid. The TSH next to it decides the story: a high TSH confirms the gland is failing, while a low or unremarkable TSH points upstream to the pituitary.
The usable supply looks adequate, but whether that is reassuring depends on the TSH beside it. A normal Free T4 sitting next to a high TSH is the textbook subclinical pattern, not an all-clear.
Points toward an overactive thyroid, usually with a suppressed TSH. High-dose biotin can fake this exact reading, so a dramatic value in someone who feels well gets a second look before anything else.
Two situations bend these cutoffs. Lab ranges for Free T4 vary more than for most markers because the assays themselves differ, so a 0.9 measured at one lab is not strictly the same result as a 0.9 at another. And pregnancy rewrites them outright: carrier proteins climb under the influence of estrogen, which raises total thyroxine and makes the free measurement harder to interpret, so obstetric care leans on pregnancy-specific ranges.
What does a low Free T4 mean?
A low Free T4 means the thyroid is putting out less usable hormone than the body is asking for. That is hypothyroidism, an underactive thyroid. What it points to next depends on the company it keeps.
The common picture is a low Free T4 alongside a high TSH. The pituitary is calling for more output and the gland can't deliver. Where dietary iodine is adequate, the usual cause is Hashimoto's disease, the autoimmune condition that slowly wears the thyroid down, which is why thyroid antibodies are often checked to confirm it. The NIDDK lists this autoimmune form as the most common cause of an underactive thyroid.
The more revealing picture is the one that catches people off guard: a low Free T4 with a TSH that is low or merely normal. Here the output is short but the pituitary isn't even raising its voice, which moves suspicion upstream, to the pituitary gland or hypothalamus rather than the thyroid itself. The American Thyroid Association notes that a pituitary abnormality can leave TSH inappropriately low while the thyroid genuinely struggles. This central form is uncommon, and that rarity is exactly why a Free T4 that disagrees with a normal TSH is worth taking seriously instead of waving away.
Severe, unrelated illness can also drag Free T4 down for a while without any thyroid disease behind it, one reason a thyroid panel drawn in the middle of an acute hospital stay can be hard to read. Because Free T4 reflects real output, it measures the depth of an underactive state in a way TSH can't: it's the number that separates a mild, compensated slowdown from overt hypothyroidism, the kind that brings the familiar fatigue, cold intolerance, weight gain, and mental fog.
What does a high Free T4 mean?
A high Free T4 means more usable hormone is in circulation than the body ordered: an overactive thyroid, or hyperthyroidism, usually with a suppressed TSH beside it. Graves' disease, the autoimmune driver, is the most common cause, but several other situations can each do it as well:
- an overactive nodule
- the early hormone-leaking phase of thyroiditis
- too large a dose of thyroid medication
- a heavy iodine load
Free T3 often runs high at the same time, and because T3 is the more active hormone, it tends to track how hard the gland is actually pushing; the Free T4 vs Free T3 comparison covers when each one leads the read.
One false alarm is worth knowing in advance. High-dose biotin, the B vitamin packed into many hair, skin, and nail supplements, interferes with the chemistry behind many thyroid assays. It can push the Free T4 result up while driving TSH down, an almost exact copy of the hyperthyroid pattern in someone whose thyroid is working normally. Laboratory medicine guidance is to stop biotin for a couple of days before thyroid testing, so a striking result paired with a supplement habit is a reason to retest, not to panic.
How to make sense of a Free T4 result
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1
Read it next to TSH, never alone
Free T4 on its own rarely settles anything; the TSH and Free T4 pair is what names the pattern. The TSH vs Free T4 comparison walks through the four combinations and what each one usually means.
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2
A low Free T4 with a normal TSH is a clue, not noise
When the two don't line up, clinicians look beyond the thyroid to the pituitary, or to a recent severe illness. It's worth flagging to your doctor rather than dismissing as a stray result.
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3
Pregnant, or on estrogen?
Birth control, hormone therapy, and pregnancy raise carrier proteins, which lifts total thyroxine and unsettles the free measurement. Ask which reference range your result was read against, since a pregnancy range differs from the standard one.
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4
On levothyroxine, expect the upper half
Clinicians often see Free T4 settle in the upper part of the range on T4 replacement even when TSH is normal, and treat that as expected. Have the blood drawn before the day's dose so the timing doesn't skew it.
Free T4 is half of a pair
Free T4 belongs beside TSH, not in place of it. The two together form the core of the thyroid panel, and the Free T4 vs Total T4 comparison explains why labs moved away from the older total measurement in routine use. For the full reading order, antibodies and all, the thyroid panel guide walks the report line by line.
Like the rest of the thyroid markers, Free T4 drifts slowly and its assays vary between labs, so a single value carries less information than its direction over time. A Free T4 that has slid from the middle of the range to the floor across two years tells a story no one in-range result can. Tracking the trend is where that story shows up.
Sources
- Thyroxine (T4) Test — MedlinePlus, National Library of Medicine
- Thyroid Function Tests — American Thyroid Association
- Hypothyroidism — NIDDK, National Institutes of Health
- Hyperthyroidism — NIDDK, National Institutes of Health
- Biotin Interference in Laboratory Tests — ADLM (AACC) Guidance
Written and reviewed by BloodSight Editorial Team · Last updated
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Normal ranges
| Group | Range | Unit |
|---|---|---|
| Adult | 0.8–1.8 | ng/dL |
Reference ranges may vary by laboratory and individual factors.
Free Thyroxine — Common Questions
What is the difference between Free T4 and Total T4?
What does a low Free T4 mean?
Can Free T4 be normal when TSH is high?
Why is my Free T4 high if I feel fine?
Does Free T4 change in pregnancy?
Disclaimer
This content is for informational and educational purposes only. It is not intended as medical advice, diagnosis, or treatment recommendation. Reference ranges may vary by laboratory. Always discuss your results with a qualified healthcare professional.
Related Tests
TSH is the most ordered thyroid test, and the most counterintuitive one to read, because the number moves in the opposite direction from your thyroid.
Free T3 is the active, finished form of thyroid hormone your tissues run on. It's also the number labs leave off most panels, and the one that drops for reasons that have nothing to do with your thyroid.
Total T4 weighs every bit of thyroxine in your blood at once, carrier proteins included. That's why the pill, pregnancy, and a quiet genetic quirk can move it while your thyroid sits perfectly still.
Reverse T3 is the inactive twin of the active thyroid hormone, the form the body parks on a siding when it wants to slow down. It is also the one thyroid test two doctors will openly disagree about ordering.
TBG is the carrier protein that holds most of your thyroid hormone in reserve. It's the number that explains why a total T4 can look abnormal while the thyroid is working perfectly.
Anti-TPO is the thyroid result that reads scariest on the page and often changes the least about today. A positive marks immune involvement and future risk, not a diagnosis you have to treat now.
Anti-thyroglobulin is the thyroid antibody whose most important job isn't describing your thyroid at all. It mostly flags whether another number on the report can be trusted.