Free T3
Active thyroid hormone triiodothyronine
Also known as
Definition
Free T3 (fT3) is the biologically active form of thyroid hormone and directly regulates metabolism, heart rate, and thermogenesis at cellular receptors. Only the unbound fraction is metabolically active, making it more informative than total T3. A low fT3 despite normal TSH may indicate a conversion problem, for example caused by selenium deficiency.
| Parameter | Value |
|---|---|
| Unit | pg/ml (pmol/l) |
| Reference Range | 2.0–4.4 pg/ml (3.1–6.8 pmol/l) |
Agree on your personal target range with your doctor.
Conversion: 1 pg/ml × 1.536 = pmol/l. US labs report pg/ml; international studies use pmol/l. Reference ranges vary by assay manufacturer.
↓ What a low value means
Low fT3 can indicate hypothyroidism, impaired T4→T3 conversion (e.g., selenium deficiency, chronic stress, caloric restriction), or Non-Thyroidal Illness Syndrome (NTIS). Symptoms: fatigue, cold intolerance, weight gain, slowed heart rate, hair loss, depressed mood, constipation. In Hashimoto's, fT3 can fall early — before TSH rises.
↑ What a high value means
Elevated fT3 points to hyperthyroidism (Graves' disease, toxic adenoma), over-dosed thyroid therapy, or rare T3-producing conditions. Symptoms: palpitations, weight loss despite good appetite, sweating, insomnia, tremor, nervousness. Isolated T3 toxicosis is possible (suppressed TSH, normal fT4, elevated fT3).
✓ What can influence this value
Free T3 is formed mainly by the conversion of T4 via deiodinases, which depend among other things on selenium and zinc. The value is influenced by thyroid diseases, iodine supply, caloric restriction, severe illness, and chronic stress (both can inhibit the conversion), as well as medications such as levothyroxine, liothyronine, amiodarone, or beta blockers. The assessment is made by the treating doctor.
When to test
Together with TSH and fT4 for thyroid complaints. Fasting is not required; the value varies over the course of the day. With levothyroxine therapy, taking a dose shortly before the blood draw can affect the result. How often it is measured is decided by your doctor.
Frequently asked questions
Why can fT3 be low even when TSH and fT4 are normal? +
T4-to-fT3 conversion occurs in peripheral tissues through deiodinase enzymes that depend on selenium and zinc. Selenium deficiency, chronic stress, or caloric restriction can inhibit this conversion — TSH and fT4 remain normal while fT3 falls.
Is fT3 measured in thyroid screening? +
Not all guidelines include fT3 as a routine test. It is measured, for example, in Hashimoto's thyroiditis, to monitor combination therapy with T3, or with persistent symptoms of an underactive thyroid despite normal TSH and fT4. Whether and when it is measured is decided by your doctor.
What factors affect T4-to-T3 conversion? +
The deiodinases that convert T4 into T3 depend among other things on selenium and zinc; iodine supply also plays a role. Caloric restriction, fasting, severe illness, and chronic stress can inhibit the conversion.
Free T3
Sources
Last updated: May 28, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.