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Free T4
The unbound, active fraction of the main thyroid hormone, always read together with TSH.
Other names: Free thyroxine, FT4Section: Thyroid
Updated: August 29, 2026·MedProtect editorial team Reference information — not a diagnosis or a prescription
In shortA typical reference range is 0.8–1.9 ng/dL (10.3–24.5 pmol/L). The range is population-based, so symptoms and the TSH result still matter when the number falls inside it.
Reference ranges
Group
Reference range
Adults
0.8–1.9 ng/dL (10.3–24.5 pmol/L)
Normal value ranges vary slightly among different laboratories, and some laboratories use different measurements or test different specimens. The range is population-based: an individual result is interpreted together with symptoms and the TSH level.
Check your value
Enter the number from your lab report — see how it compares with the typical range. Mind the units shown in the chosen row.
A rough comparison with a typical range, not a diagnosis. Reference ranges vary between laboratories — check the range on your own report and let a clinician interpret the result.
What this marker is
T4 (thyroxine) is the main hormone produced by the thyroid gland. Most of it travels through the blood attached to carrier proteins, and the small unbound fraction — free T4 — is what this test measures. Because the free fraction is the part available to the body's tissues, it reflects thyroid function more directly than the total amount.
The test is recommended when other thyroid blood tests such as TSH or T3 come back abnormal, when there are symptoms of an overactive or underactive thyroid, when the pituitary gland may not be producing enough of its hormones (hypopituitarism), when a lump or enlargement of the thyroid is found, and in some cases of difficulty becoming pregnant. It is also used to monitor people being treated for thyroid problems.
Results are reported in nanograms per deciliter (ng/dL); in picomoles per liter (pmol/L) in many countries. A typical normal range is 0.8–1.9 ng/dL (10.3–24.5 pmol/L), but the result is never read alone: to fully understand a free T4 value, the results of other thyroid tests, such as TSH or T3, may be needed.
The normal range is based on a large population and is not necessarily normal for an individual — symptoms of hyperthyroidism or hypothyroidism can occur even while free T4 sits inside the range, and the TSH test may help determine whether symptoms are related to thyroid disease. Test results can also be affected by pregnancy, estrogen level, liver problems, more severe body-wide illnesses, and inherited changes in the protein that binds T4.
When it is high
Graves diseaseThe most common overactive-thyroid condition: the immune system stimulates the thyroid, and free T4 rises.
Too much thyroid hormone medicineAn excessive dose of thyroid hormone replacement pushes free T4 above the range.
ThyroiditisInflammation of the thyroid releases stored hormone into the blood.
Toxic goiter or toxic thyroid nodulesAutonomously working thyroid tissue produces hormone without waiting for the TSH signal.
Rare causesSome tumors of the testes or ovaries (rare), medical imaging with iodine-containing contrast dye (rare, and only if there is a thyroid problem), and eating a lot of iodine-rich foods (very rare, and only if there is a thyroid problem).
When it is low
HypothyroidismAn underactive thyroid — including Hashimoto disease and other disorders involving reduced thyroid function — lowers free T4.
Severe acute illnessA serious body-wide illness can temporarily push free T4 down without primary thyroid disease.
Malnutrition or fastingInadequate nutrition reduces thyroid hormone production.
Certain medicinesSome medicines lower free T4, which is why the provider reviews the medication list when interpreting the result.
What can skew the result
These factors can shift the value without any disease — worth considering before a retest:
Biotin (vitamin B7) supplementsPregnancyKidney and liver diseaseEstrogen levelSevere body-wide illnessInherited changes in the protein that binds T4
How to prepare
No special preparation is needed. In general, test results are not affected by medicines, but biotin (vitamin B7) can affect them — the provider needs to know if it is being taken. Pregnancy and kidney or liver disease can also affect the result.
When a retest makes sense
A free T4 result is interpreted together with TSH and, when needed, T3 — the combination separates thyroid causes from pituitary ones. During treatment for a thyroid problem, the test is repeated on the schedule set by the treating provider to track the response.
Who usually reviews this result
A result outside the reference range is usually reviewed by: an endocrinologist; your GP or primary care doctor for the first evaluation. Bring your lab report and any previous results to the appointment.
Names used on lab reports
Your report may use any of these names: Free T4T4Free thyroxineFT4Thyroxine test by equilibrium dialysis
Units and conversion
Measured in: ng/dL
ng/dL=pmol/L
ng/dL × 12.87 = pmol/L. Example: 1.0 ng/dL = 12.9 pmol/L. Results are rounded to 2 decimal places.
Several markers on your report?
See how MedProtect explains results in the app: the whole report, a check of the recognized values, and a clear breakdown.
In general they do not, but biotin (vitamin B7) can affect the results. Pregnancy, estrogen level, liver problems, severe illness, and inherited changes in the T4-binding protein can also shift the number.
Can free T4 be normal while symptoms persist?
Yes. The normal range is based on a large population and is not necessarily normal for an individual. Symptoms of hyperthyroidism or hypothyroidism can occur with an in-range free T4, and the TSH test may then help determine whether the thyroid is involved.
What is the difference between free T4 and total T4?
Total T4 counts all thyroxine in the blood, bound and unbound; free T4 counts only the unbound, active fraction. The free fraction is less affected by changes in binding proteins, which is why it is the preferred measurement.
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