Free triiodothyronine (free T3) is the most active hormone of the thyroid gland. It does the main work in the tissues: it speeds up metabolism and affects the heart, body temperature and the nervous system.
Most T3 is formed not in the gland itself but in the tissues — from thyroxine (T4), which "matures" for this. So free T3 reflects not only hormone production but also the quality of its peripheral conversion — and is always interpreted together with TSH and free T4.
Reference ranges
The reference depends on the lab and method; a typical adult range:
| Adults (typical reference) | 3.1 – 6.8 pmol/L |
| T3 thyrotoxicosis | free T3 elevated with normal free T4 |
| Hyperthyroidism | free T3 and T4 elevated, TSH reduced |
| Hypothyroidism | free T3 usually normal, falls late |
| Low-T3 syndrome | free T3 reduced in severe illness (TSH and free T4 normal) |
What an elevated level means
Elevated free T3 means an excess of active hormone — thyrotoxicosis, usually with suppressed TSH.
Sometimes there is T3 thyrotoxicosis: free T3 is already elevated while free T4 is still normal — an early or isolated variant of hyperthyroidism (for example, with toxic nodular goiter or at the onset of Graves' disease).
Symptoms: palpitations, tremor, weight loss, sweating, anxiety, heat intolerance.
What a low level means
Reduced free T3 occurs in two fundamentally different situations.
1. Hypothyroidism. But with a hormone shortfall free T3 falls last: the body maintains the active hormone level to the end, so TSH and free T4 are more important for diagnosing hypothyroidism.
2. Low-T3 syndrome (euthyroid sick syndrome). In severe illness, fasting or after surgery the body deliberately reduces T4-to-T3 conversion to save energy. The thyroid is healthy here — such "low T3" does not need to be treated with hormones.
How the test is done
Free T3 is drawn from a vein; strict fasting is not required; biotin (vitamin B7) is stopped 2–3 days before. Blood is taken in the morning.
Free T3 is not used as a first or sole test. It is ordered selectively — more often when hyperthyroidism is suspected (including T3 thyrotoxicosis) or to assess T4→T3 conversion — and is always interpreted together with TSH and free T4.
What affects the value
- Biotin (vitamin B7) in large doses distorts the result — stop it 2–3 days before.
- Severe acute illness, fasting, stress — reduce T4→T3 conversion (low-T3 syndrome).
- Medications: beta-blockers, amiodarone, glucocorticoids, propylthiouracil — inhibit conversion.
- Selenium and zinc deficiency — cofactors of the enzyme that converts T4 to T3.
- Pregnancy — norms differ.
- Lab method — direct and calculated free T3 may differ.
Assess the thyroid
Calculators of thyroid function and peripheral T4→T3 conversion:
Frequently asked questions
How does free T3 differ from free T4?
Free T4 (thyroxine) is a prohormone, a "store", while free T3 is the active form that actually works in the tissues. Most T3 is made from T4 already in the tissues. So free T4 tells about the gland's hormone production, and free T3 also about the quality of its conversion into the active form.
Free T3 is normal but TSH is elevated — what to do?
This is a typical picture of subclinical or early hypothyroidism: the body maintains active T3 in range the longest. Go by TSH and free T4, not free T3 — in hypothyroidism it falls last.
Why is free T3 low if the thyroid is healthy?
Most likely this is low-T3 syndrome (euthyroid sick syndrome): in severe illness, surgery or fasting the body deliberately reduces T4-to-T3 conversion. This is a protective response, not a thyroid disease — it does not need to be treated with thyroid hormones.
Related terms
Sources
- MedlinePlus (NIH) — Triiodothyronine (T3) Tests
- American Thyroid Association — Thyroid Function Tests
- Warner M.H., Beckett G.J. Low T3 syndrome (review). NCBI/PMC