Free T4 measures the portion of thyroxine in the blood that is not bound to carrier proteins. It is ordered with TSH to assess thyroid function, and it is more informative than total T4 when binding proteins change. It does not confirm or rule out a condition on its own, and the range printed on your report applies.
Most of the thyroxine in the blood is carried by proteins, and only a small fraction circulates unbound. That unbound fraction is what the test reports, which is why the result is called free T4 rather than total T4. The distinction matters whenever the carrier proteins shift.
What the test measures
Thyroxine, or T4, is the main hormone the thyroid gland produces. It travels in the blood mostly attached to thyroid-binding globulin, albumin, and transthyretin, with a small unbound fraction in equilibrium with the bound pool. Free T4 is that unbound fraction, the part available to enter tissues and be converted to the active hormone T3.
Laboratories estimate the free fraction with an immunoassay, and some methods separate the free hormone more directly before measurement. The methods do not always agree, particularly when binding proteins are unusual, which is one reason the printed range belongs to the method that produced it.
Some reports print total T4 as well. Total T4 measures bound and free hormone together, so it moves when the carrier proteins change even if the free fraction does not.
Why it may be ordered
- With TSH, as part of a thyroid profile or a general health checkup.
- When the TSH result needs context, because free T4 shows what the gland itself is producing.
- To follow a known thyroid condition over time.
- During pregnancy, when binding proteins rise and the interpretation of thyroid results shifts.
- When a total T4 result looks out of step with the clinical picture, for example when binding proteins are altered.
The test is not a universal screening test, and it is not a substitute for a clinical assessment. A free T4 result is read with TSH, and often with free T3 and TPO antibodies.
The unit on your report
| What you may see | What it means |
|---|---|
| Free T4, FT4, free thyroxine | The test name. Laboratories abbreviate it differently. |
| ng/dL | Nanograms per decilitre, a mass unit common on reports in India and the United States. |
| pmol/L | Picomoles per litre, a molar unit common on reports in the United Kingdom and parts of Europe. |
| Total T4, TT4 | A different test that measures bound and free thyroxine together. |
| Free T4 index, FTI | A calculated estimate some laboratories derive from total T4 and a binding measurement. It is not the same measurement as free T4. |
Why this page does not print a range
Free T4 ranges are method-specific. Immunoassays from different manufacturers can produce different values on the same sample, and the reference interval is established for the assay and the population the laboratory serves. Age, sex, pregnancy, and the clinical context can shift what a laboratory considers its reference interval.
The range printed on the report that produced your value is the one that applies. Doctano keeps that printed range with the result and does not substitute a range from the internet.
What a high value can mean
A higher free T4 means more unbound thyroxine is circulating. It can be associated with a thyroid gland producing or releasing more hormone than usual, and it can occur with thyroid hormone taken as a medicine. A clinician reads it with TSH, because the two usually move in opposite directions when the thyroid itself is the source.
Some medicines change protein binding and can raise the free T4 estimate without a change in thyroid function. Assay interference and severe illness can also produce an unexpected value.
What a low value can mean
A lower free T4 means less unbound thyroxine is circulating. It can be associated with a thyroid gland producing less hormone than the body needs, and it can also occur when the pituitary or hypothalamus is not signalling the gland adequately. In that situation TSH is usually low or inappropriately unremarkable rather than high.
Severe illness, some medicines, and pregnancy can lower free T4 or change how it is measured. A low value is therefore read with TSH and the history, and a result inside the printed range is not proof that the thyroid axis is entirely settled.
What can move the value
- Binding proteins. Pregnancy, oestrogen-containing medicines, and some liver and kidney conditions change the proteins that carry thyroxine.
- Pregnancy. Binding proteins rise and the laboratory may use a range suited to pregnancy.
- Medicines. Thyroid hormone raises free T4, and some anticonvulsants and other medicines alter binding or metabolism.
- Severe illness. Acute illness can change thyroid hormone results in ways that do not reflect the gland's usual output.
- Assay method. Immunoassays and separation methods can disagree, especially when binding is unusual.
- Sample handling. Delayed processing or haemolysis can affect some assays.
What to compare it with
Read free T4 with TSH first. When both are printed, the pair shows both the pituitary signal and the gland's output. Free T3 adds the active hormone, and TPO antibodies add information about an autoimmune contribution. Where a total T4 is also printed, treat it as a different measurement rather than a second opinion on the same one.
The most useful comparison is the previous free T4 from the same laboratory and method. Values from different assays are not automatically comparable, even in the same unit.
How to see the trend
Doctano reads the free T4 value, its unit, and the reference range printed on the report. It keeps the range with the result, normalizes the marker name across laboratories, and places the value on the profile's timeline. Each point still opens the report it came from, so the original page can be checked.
When reports use different units, Doctano normalizes them for the timeline so the points line up. Verify the value and the unit against the original report before treating a change as real, especially when the method or the laboratory changed.
What this page is not
This page explains the test in general. It does not interpret your result, diagnose a condition, or recommend treatment. It is not medical advice. Verify the value and the unit against the original report, and discuss the result with a qualified professional.