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Free T3
The unbound fraction of triiodothyronine, the hormone that sets the pace of metabolism.
Other names: Free triiodothyronine, FT3Section: Thyroid
Updated: August 29, 2026·MedProtect editorial team Reference information — not a diagnosis or a prescription
In shortA typical free T3 reference range is 2.1–4.4 pg/mL (3.2–6.8 pmol/L). For people under 20, normal values are age-specific.
Reference ranges
Group
Reference range
Adults, free T3
2.1–4.4 pg/mL (3.2–6.8 pmol/L)
People under 20
Age-specific ranges — checked against the laboratory's table
Normal value ranges vary slightly among different laboratories. The same range may be reported as 210–440 pg/dL. Free T3 assays are generally less accurate than total T3 assays.
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
Triiodothyronine (T3) is a thyroid hormone that plays an important role in the body's control of metabolism — the many processes that set the rate of activity in cells and tissues. Thyroid function depends on the action of T3 together with other hormones, including thyroid-stimulating hormone (TSH) and T4, so the three tests are often evaluated together.
Most T3 in the blood is attached to proteins, and the rest floats free. The total T3 test measures both fractions; the free T3 test measures only the unbound hormone. Tests for free T3 are generally less accurate than tests for total T3, which is worth keeping in mind when a result looks unexpected.
Results are reported in picograms per milliliter (pg/mL) — the same numbers appear as 210–440 pg/dL on some reports — and in picomoles per liter (pmol/L) in many countries. The typical reference range is 2.1–4.4 pg/mL (3.2–6.8 pmol/L). Normal values are age-specific for people younger than 20, so a child's or teenager's result is checked against the corresponding age range.
The provider may recommend this test when there are signs of a thyroid disorder: an overactive or underactive thyroid, a pituitary gland that does not produce normal amounts of its hormones (hypopituitarism), or during treatment for hypothyroidism. Several medicines shift T3 measurements in either direction, so the medication list is part of the interpretation.
When it is high
Overactive thyroid glandA higher-than-normal T3 may be a sign of an overactive thyroid, for example in Graves disease.
T3 thyrotoxicosisA rare form of thyrotoxicosis in which T3 rises while other thyroid measurements may still look unremarkable.
Toxic nodular goiterAutonomously functioning thyroid nodules produce excess hormone.
Thyroid medicines and certain supplementsA common cause — taking thyroid hormone preparations or supplements that contain thyroid hormone raises T3.
Liver diseaseThe liver takes part in thyroid hormone metabolism, and its diseases can push T3 up.
Pregnancy and estrogensA high T3 may occur in pregnancy, especially with morning sickness at the end of the first trimester, or with the use of birth control pills or estrogen.
When it is low
Severe illnessSevere short-term illnesses and some long-term illnesses lower T3 — a pattern that does not by itself mean thyroid disease.
ThyroiditisInflammation of the thyroid gland lowers hormone output; Hashimoto disease is the most common type.
StarvationSevere calorie restriction reduces the conversion of T4 to T3.
Underactive thyroid glandIn hypothyroidism the thyroid produces less hormone, and T3 falls.
Selenium deficiencySelenium deficiency decreases the conversion of T4 to T3, although it is not clear that this results in lower-than-normal T3 levels in people.
What can skew the result
These factors can shift the value without any disease — worth considering before a retest:
Medicines that raise T3: birth control pills, estrogens, methadone, certain herbal remediesMedicines that lower T3: amiodarone, anabolic steroids, androgens, antithyroid medicines such as propylthiouracil and methimazole, lithium, phenytoin, propranololPregnancy, especially with morning sickness at the end of the first trimesterSevere illness and starvation
How to prepare
The provider decides whether any medicines should be paused before the test, and nothing is stopped without that advice. Birth control pills, estrogens, methadone, and certain herbal remedies can raise T3 measurements, while amiodarone, anabolic steroids, androgens, antithyroid medicines, lithium, phenytoin, and propranolol can lower them.
When a retest makes sense
A free T3 result is interpreted together with TSH and free T4, and sometimes with the total T3 measurement, which is generally more accurate. During treatment for a thyroid disorder, the treating provider sets the retesting schedule.
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 T3T3FT3TriiodothyronineT3 radioimmunoassay
Units and conversion
Measured in: pg/mL
pg/mL=pmol/L
pg/mL × 1.536 = pmol/L. Example: 2.1 pg/mL = 3.2 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.
Free T3 reflects the unbound, active fraction of the hormone, and in some situations — for example suspected T3 thyrotoxicosis — it adds information that total T3 does not. When precision matters most, the provider may rely on the total T3 assay instead.
Which medicines affect the result?
Birth control pills, estrogens, methadone, and certain herbal remedies can increase T3 measurements. Amiodarone, anabolic steroids, androgens, antithyroid medicines such as propylthiouracil and methimazole, lithium, phenytoin, and propranolol can decrease them.
Are the ranges the same for children?
No. Normal values are age-specific for people younger than 20, so the result is checked against the laboratory's age-matched table rather than the adult range.
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