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 (2.6–5.7 pmol/l) |
| Optimal Range (lab2go) | 3.0–4.2 pg/ml (lab2go Optimum Zone) |
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).
✓ How to optimize this marker
For low fT3 with normal TSH and fT4: selenium (100–200 µg/day as selenomethionine) supports deiodinase activity for T4→T3 conversion. Correct zinc deficiency. Reduce caloric restriction and chronic stress (both impair conversion). Ensure adequate iodine (iodized salt, seafood). Do not self-medicate with T3 without medical workup.
When to test
Together with TSH and fT4 for thyroid complaints. No fasting required, but morning collection (8–10 am) is recommended for diurnal variation. With levothyroxine therapy: blood draw before morning dose. Recheck 6–8 weeks after therapy change.
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.
Should fT3 be measured in thyroid screening? +
Not recommended in all guidelines as routine, but useful for persistent hypothyroid symptoms despite normal TSH/fT4, in Hashimoto's thyroiditis, and for monitoring combination therapy with T3.
What factors improve T4-to-T3 conversion? +
Selenium-rich foods (Brazil nuts, seafood), adequate zinc, moderate iodine, stress reduction, and adequate sleep. Excessive caloric restriction, fasting, and chronic stress inhibit conversion.
Sources
Last Reviewed: May 28, 2026 · sina
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.