Thyroxine, Free (FT4)
Free T4 measures the active, unbound form of the main thyroid hormone. Along with TSH, it is essential for diagnosing thyroid disorders and adjusting thyroid medication dosage.
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What the Thyroxine, Free (FT4) test tells you
Thyroxine, or T4, is the main hormone the thyroid gland releases into the bloodstream. Almost all of it travels attached to carrier proteins, and only the small unbound fraction can enter cells and act. The free T4 test measures that active fraction specifically, which is why it gives a truer picture of thyroid activity than a measurement of total thyroxine.
T4 is best thought of as a reservoir hormone. It is relatively stable in the circulation and is converted, mainly in the liver, kidneys and other tissues, into the more potent T3 at the point where it is needed. Because the free fraction is not influenced by changes in carrier protein levels, free T4 stays interpretable in situations that distort total T4: pregnancy, oestrogen-containing medication, protein loss through the kidneys, and chronic liver disease among them. That reliability is the reason it, rather than total T4, is the routine partner to TSH.
Free T4 is requested on its own when the specific question is how much usable hormone the thyroid is putting out right now. It is read together with TSH rather than alone, and the combination is what carries the meaning. TSH and free T4 moving in opposite directions points towards a problem in the thyroid gland itself; both being low together raises the question of a pituitary cause; both being high is unusual and prompts a search for an uncommon explanation. Free T4 is also the measurement that responds most directly to a change in levothyroxine dose, which makes it useful in the first weeks of treatment before TSH has caught up.
Essential companion to TSH for diagnosing and managing hypothyroidism and hyperthyroidism. Also important for monitoring thyroid hormone replacement.
- Measuring active hormone
Free T4 reflects the fraction of thyroxine actually available to your tissues, unaffected by the carrier proteins that distort a total measurement.
- Characterising an abnormal TSH
When TSH comes back outside the usual pattern, free T4 shows whether thyroid hormone output has genuinely changed and in which direction.
- Locating the problem
Read alongside TSH, free T4 helps separate a thyroid gland problem from a pituitary one, which changes the whole approach to treatment.
- Guiding early dose changes
Free T4 responds to a change in levothyroxine or anti-thyroid medication sooner than TSH does, so it is often the more useful figure early on.
Reasons your doctor may request the Thyroxine, Free (FT4) test
Your doctor may order this test for any of the reasons below. Some are routine checks, others follow specific symptoms or an earlier result.
Symptoms of an under- or overactive thyroid
- Symptoms of an overactive thyroid such as palpitations, tremor, heat intolerance or weight loss.
- Symptoms of an underactive thyroid such as fatigue, cold intolerance, constipation or dry skin.
Characterising an abnormal TSH
- An abnormal TSH result that needs to be characterised.
- Suspected pituitary disease, where TSH alone cannot be relied on.
Early treatment, pregnancy & fertility care
- Monitoring the first months of levothyroxine or anti-thyroid drug treatment, before TSH has caught up.
- Thyroid assessment during pregnancy or fertility care.
A result in the Normal band, 0.9 to 1.7 ng/dL, means the thyroid is putting out a usual amount of usable hormone. A result in the Low band, 0 to 0.8 ng/dL, is most often an underactive gland (autoimmune thyroiditis, surgery or radioactive iodine) or levothyroxine taken at too low a dose or inconsistently; severe illness and medicines such as phenytoin and corticosteroids lower it too. A result in the High band, 1.8 to 4 ng/dL, suggests an overactive gland, the release phase of thyroiditis, or more replacement than your body requires. Free T4 carries its meaning next to TSH rather than alone: the two moving in opposite directions points to the thyroid itself, while both being low raises the question of a pituitary cause. Biotin supplements interfere with the assay, and methods differ, so monitor within one laboratory. Your doctor reads it with your TSH, medication and symptoms.
Medically reviewed by Dr Nirupama Sabhapathy
Ranges are a guide: your physician interprets your result alongside your symptoms and history.
What a higher result can point to
- An overactive thyroid gland, most often Graves' disease or an autonomously functioning nodule.
- The release phase of thyroiditis, when stored hormone leaks from an inflamed gland.
- Thyroid hormone replacement at more than the body requires.
- Amiodarone and other iodine-rich drugs, which can drive the thyroid into overactivity.
- Rarely, a TSH-producing pituitary tumour, or reduced tissue sensitivity to thyroid hormone.
What a lower result can point to
- An underactive thyroid from autoimmune thyroiditis, thyroid surgery or radioactive iodine.
- Insufficient or inconsistently taken levothyroxine.
- Pituitary or hypothalamic disease, where the thyroid is not being stimulated enough.
- Severe non-thyroidal illness, which lowers measured thyroid hormones without thyroid disease.
- Medicines including phenytoin, carbamazepine and high-dose corticosteroids.
These are possibilities, not diagnoses. A single value outside the reference range is interpreted alongside your symptoms, history and other tests: your doctor decides what it means for you.
What this test cannot tell you
- Free T4 alone cannot distinguish a thyroid problem from a pituitary one; TSH is needed alongside it.
- Laboratory methods are not interchangeable, so serial monitoring is best done in one laboratory.
- Acute illness, and drugs that displace hormone from its carrier proteins, can shift the value without thyroid disease being present.
- High-dose biotin supplements interfere with the assay and can produce a falsely abnormal figure.
Risks of the blood draw
- Mild bruising or brief soreness where the needle went in.
- Occasional lightheadedness, which settles after a few minutes of rest.
- Bleeding or infection at the site, which is rare with sterile single-use equipment.
- 1Free T4 measures the unbound, biologically usable fraction of thyroxine.
- 2Unlike total T4, it is not affected by changes in carrier proteins.
- 3Its meaning comes from being read together with TSH, not on its own.
- 4It reacts to a dose change faster than TSH, making it useful early in treatment.
- 5T4 acts largely as a reservoir that tissues convert into the more active T3.
Thyroxine, Free (FT4) is also part of these health checkups, which add a wider screen in the same appointment.
Thyroxine, Free (FT4) also comes in these focused packages, which pair it with the tests usually read alongside it.
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Frequently Asked Questions
Everything you need to know about the Thyroxine, Free (FT4) test. Can't find what you're looking for?
Talk to our team→Our laboratory reports 0.9 to 1.7 ng/dL as normal, meaning the thyroid is putting out a usual amount of usable hormone. A result of 0 to 0.8 ng/dL is reported as low and 1.8 to 4 ng/dL as high. Methods differ between laboratories, so if your levels are being monitored, keep to one laboratory.
A low free T4 most often points to an underactive thyroid, from autoimmune thyroiditis, thyroid surgery or radioactive iodine. In someone already on levothyroxine, it can mean the dose is too low or the tablets are being taken inconsistently. Your doctor reads it with TSH: a high TSH points to the thyroid itself, while a low TSH alongside it raises the question of a pituitary cause.
A high free T4 suggests an overactive thyroid, most often Graves' disease or a nodule producing hormone on its own, or the release phase of thyroiditis. In people on thyroid hormone replacement it can mean the dose is more than the body requires. Amiodarone and other iodine-rich drugs can also cause it. Your doctor confirms the picture with TSH and your symptoms.
Free T4 carries its meaning next to TSH rather than alone. When the two move in opposite directions, the problem usually lies in the thyroid itself; when both are low, a pituitary cause comes into question. TSH is the more sensitive screen, while free T4 shows how much usable hormone is actually circulating and reacts to a dose change faster.
Yes. Biotin and high-dose B-complex supplements interfere with the assay and can produce a falsely abnormal figure that looks like a thyroid problem. They are usually stopped for a few days before the test on your doctor's advice, so mention any hair, skin, nail or vitamin product you take.
Total T4 counts every thyroxine molecule in the sample, including the great majority that is attached to carrier proteins and inactive. Free T4 counts only the unbound portion your cells can actually use, which is why it stays meaningful when carrier protein levels change.
Free T4 reflects what the thyroid is releasing right now, whereas TSH reflects the pituitary's slower reassessment of the situation. Early in treatment the two can disagree for a while, and that is expected.
Yes, briefly. A dose taken shortly before the draw raises free T4 for a number of hours, so the sample is best taken before that day's tablet.
It can. Serious illness, some anticonvulsants and pituitary disease all lower free T4 by mechanisms that have nothing to do with the thyroid gland, which is why the result is never read in isolation.
No fasting is needed. If you take levothyroxine, take that day's dose after your blood is drawn, because the dose briefly raises free T4 and can make a well-adjusted dose look excessive.
The free T4 test is AED 35, including a physician-reviewed report, with results usually within 24 hours. Home collection anywhere in Dubai is free when your order reaches AED 300 or includes a package; below that, a AED 100 home collection fee applies (AED 50 for a lab walk-in).