- 01Free T4 is read with TSH, never alone
One number cannot tell you what is wrong. A high TSH means overt hypothyroidism when free T4 is low, but only the milder subclinical form when free T4 is still normal, and a low TSH sitting with a low free T4 flips the story to the pituitary. Free T4 does not screen, TSH does that; free T4 comes in second to confirm an abnormal TSH, grade how far off things are, and catch the pituitary pattern TSH would miss. You have to read both numbers together to know what either one means.
- 02'Free' beats 'total' because carrier proteins move
Total T4 rises and falls with the carrier proteins that go up in pregnancy or on estrogen, so it can read abnormal with a healthy thyroid. Free T4 measures only the unbound, usable slice, so it tracks thyroid status rather than protein levels.
Free T4 measures the unbound, usable form of thyroxine (T4), the main hormone your thyroid makes, and it earns its meaning only next to your TSH, the pituitary gland's signal telling the thyroid to speed up or slow down. Read as a pair, the two numbers sort out how far off your thyroid is and where the fault sits. A high TSH with a low free T4 is overt hypothyroidism, a clearly underactive thyroid; a high TSH with a still-normal free T4 is the milder, earlier subclinical version; a low TSH with a high free T4 points the other way, to an overactive thyroid.
What free T4 actually measures
Your thyroid mostly makes thyroxine, or T4. It is roughly 80 to 90% of what the gland secretes, and it works as a prohormone, a hormone that stays largely inactive until tissues convert it into the more potent T3. 1 Almost all of that T4 travels through your blood stuck to carrier proteins. Only a tiny unbound slice, about 0.03%, floats free, and that free slice is the part your cells can take up and use. 4 A free T4 test measures that unbound fraction.
Older tests reported total T4, every bit of thyroxine in the blood, bound and free together. The problem is that the carrier proteins themselves move. Pregnancy, estrogen therapy and the combined pill raise them; some illnesses lower them. When the proteins rise, total T4 rises too, even though the thyroid has not changed. 4 Free T4 sees past that, which is why labs report it and clinicians read it.
Free T4 vs total T4: which number to trust
What is a normal free T4 level?
A typical adult free T4 range is about 0.8 to 1.8 ng/dL (roughly 10 to 23 pmol/L), but the exact cutoffs depend on the assay (the specific lab test method) your lab runs, so read your result against the range printed on your own report. 4 The number on its own does not diagnose anything; what it means depends on the TSH it sits beside.
| Free T4 | Typical value | What it points to (always read with TSH) |
|---|---|---|
| Low | below about 0.8 ng/dL | Underactive thyroid if TSH is high (overt hypothyroidism); a pituitary cause if TSH is low or normal (central hypothyroidism) |
| Normal | about 0.8 to 1.8 ng/dL | Output is holding up; if TSH is high, the early subclinical pattern; if TSH is low, early or T3-driven overactivity |
| High | above about 1.8 ng/dL | Overactive thyroid (hyperthyroidism) if TSH is low; rarer pituitary or lab-interference causes if TSH is not suppressed |
To see how that plays out, here is where one example free T4 falls against those cutoffs.
Where an example free T4 fallsng/dL
Free T4 vs TSH: why both are on the report
TSH is the first test because it is the most sensitive. TSH and free T4 have an inverse, log-linear relationship: as free T4 drifts down, TSH climbs steeply, so a small fall in thyroid hormone shows up as a large rise in TSH. 3 That sensitivity is why TSH is the recommended first-line screen for thyroid problems, the test to run before any other. 3
TSH is the best and, oftentimes, only test needed.Michael T. Sheehan, Clinical Medicine & Research, 2016
Free T4 is what you add when TSH is abnormal or when TSH alone could mislead. It confirms an abnormal screen, grades how far off things are, and catches the one pattern TSH cannot flag alone, a pituitary problem where a low TSH sits with a low free T4. To see how every number on the report fits together, the how to read your thyroid panel guide walks through the whole panel, marker by marker.
Reading free T4 with TSH: the patterns
Because the two numbers move together in a predictable way, the pair lands you in one of a few boxes. Here is what each combination usually points to. None of these is a diagnosis on its own; each is a direction for a clinician to confirm. 2
| Pattern | TSH | Free T4 | What it usually points to |
|---|---|---|---|
| Normal | Normal | Normal | No thyroid problem on these two numbers |
| Subclinical hypothyroidism | High | Normal | The early, mild form: the pituitary is pushing harder but hormone output still holds |
| Overt hypothyroidism | High | Low | A clearly underactive thyroid |
| Central hypothyroidism | Low or normal | Low | A pituitary or hypothalamic cause, the pattern TSH alone would miss |
| Hyperthyroidism | Low | High or normal | An overactive thyroid; free T3 grades it and catches T3-driven cases |
What a low free T4 means
A low free T4 means less thyroid hormone is reaching your tissues. Paired with a high TSH, it is overt hypothyroidism, an openly underactive thyroid, and the usual next step is a clinician's review to look for the cause and decide on treatment. Paired with a low or unexpectedly normal TSH, a low free T4 is the flag for central hypothyroidism, where the pituitary gland, or the hypothalamus above it, is not sending enough signal; this is the pattern TSH would miss on its own. 4 Either way, a low free T4 belongs in front of a clinician, alongside your TSH. For the condition a low free T4 with a high TSH usually points to, see hypothyroidism, the underactive-thyroid state where the gland cannot keep up with the body's demand.
What a high free T4 means
A high free T4 means too much thyroid hormone. With a suppressed (low) TSH, that is hyperthyroidism, an overactive thyroid. Free T4 grades how severe it is, and its sibling marker free T3, the more active hormone your tissues make mostly by converting T4, confirms it and can catch T3-driven cases where free T4 still reads normal. A high free T4 with a TSH that is not low is unusual and points to rarer causes or lab interference, which a clinician should sort out.
Subclinical vs overt: same high TSH, different free T4
The single most useful thing free T4 does is split a high TSH into two different situations. Both have a high TSH. In subclinical hypothyroidism the free T4 is still in range: the pituitary is pushing harder, but the thyroid is keeping output up, so it is the early, mild stage, often without clear symptoms. In overt hypothyroidism the free T4 has fallen below range: the thyroid can no longer keep up. 2 Same alarm on the TSH, different severity, and free T4 is the number that tells them apart.
How free T4 is tested, and how often
Free T4 is a single blood draw, usually from a vein in your arm. You do not need to fast for it, and it does not have to be a separate appointment: it is part of a standard thyroid panel and is often run automatically, as a reflex test, when a screening TSH comes back abnormal. If your TSH is normal and you have no symptoms, most people do not need free T4 added. When a result is off, retesting is usually done after a few weeks rather than the same week, because thyroid numbers shift slowly and single readings can wobble; your clinician sets the timing from the pattern.
Citations
- Jonklaas J, et al. Guidelines for the Treatment of Hypothyroidism (2014 American Thyroid Association Task Force). Thyroid. 2014;24(12):1670-1751.
- Garber JR, et al. Clinical Practice Guidelines for Hypothyroidism in Adults (AACE/ATA). Thyroid. 2012;22(12):1200-1235.
- Sheehan MT. Biochemical Testing of the Thyroid: TSH is the Best and, Oftentimes, Only Test Needed. Clin Med Res. 2016;14(2):83-92.
- Koulouri O, et al. Pitfalls in the measurement and interpretation of thyroid function tests. Best Pract Res Clin Endocrinol Metab. 2013;27(6):745-762.
- Alexander EK, et al. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid. 2017;27(3):315-389.
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.
Associations to explore with a clinician, not a diagnosis from a single number.