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Intel Lab Diagnostics Dubai
Single Biomarker

Thyroxine, Free (FT4)

Free Thyroid Hormone (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.

BLOOD24 HOURSNO FASTING
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  • Free home collection
  • Physician-reviewed report

24 Hours

Turnaround

Blood

Sample

About This Test

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.

Sample type
Blood
Measured in
ng/dL
Turnaround
24 Hours
Fasting
Not required
Best for
Adults
Why Get Tested?

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.

When This Test Is Ordered

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.
Preparation
The short version
No prep needed.
  • Blood
  • Free home collection
  • No fasting
  • Results in 24 Hours
  1. 1
    Book any time

    No fasting window: pick any slot that suits you, online or on WhatsApp.

  2. 2
    We come to you

    A DHA-licensed nurse collects your sample at home or in the office: free home collection.

  3. 3
    Results in 24 Hours

    A physician-reviewed, easy-to-read report lands securely by WhatsApp and email.

Prep note
Physician reviewed

No fasting is needed.

If you take levothyroxine, take the day's dose after your blood is drawn: absorption produces a temporary peak in free T4 for some hours afterwards that can make a well-adjusted dose look excessive. Keep the timing of monitoring samples reasonably consistent.

Biotin and high-dose B-complex supplements interfere with the assay and are usually stopped for a few days beforehand on your doctor's advice. Mention pregnancy, oestrogen-containing medication, amiodarone, phenytoin, carbamazepine, heparin or recent contrast imaging, and postpone routine testing until you have recovered from any acute illness.

Understanding Your Results

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.

SmartReport
0.81.7
Healthy zone1ng/dLNormal
Reference ranges · ng/dL
Low
Low FT4: may indicate an underactive thyroid.
0–0.8
Normal
Normal thyroid hormone level.
0.9–1.7
High
High FT4: may indicate an overactive thyroid.
1.8–4

Ranges are a guide: your physician interprets your result alongside your symptoms and history.

Reading a High or Low Result

  • 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.

Limitations & Safety

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.
Key Points
  • 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.
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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?

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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.