What this test measures
Triiodothyronine (T3) is the more biologically active thyroid hormone — roughly 3–4× the potency of T4 at the nuclear receptor. About 80% of circulating T3 comes from peripheral conversion of T4 by deiodinase enzymes, with only 20% from direct thyroid secretion. As with T4, most is bound to carrier proteins; only the free fraction (FT3, ~ 0.3%) is bioavailable.
The FT3 assay measures this unbound fraction. Reference ranges are tighter than those of total T3 because FT3 is not affected by binding-protein changes.
Why it matters
FT3 is most useful in hyperthyroidism. In early Graves' disease or "T3 toxicosis," FT3 may rise before FT4 — so an isolated raised FT3 with a suppressed TSH should not be dismissed. FT3 is also useful in monitoring antithyroid drug treatment in Graves' (FT3 often takes longer to normalise than FT4).
In hypothyroidism FT3 adds less, because T3 is preserved at the expense of T4 until late in the disease. In acute illness, FT3 falls first (sick euthyroid syndrome) — interpret cautiously and repeat after recovery. Drugs that block T4-to-T3 conversion (propranolol, amiodarone, high-dose steroids) also lower FT3 without true hypothyroidism.
How to prepare
No fasting required. Morning sample preferred. If on levothyroxine or liothyronine, take the dose after the draw. Stop biotin / cosmetic supplements 48–72 hours before. Continue other medications.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Free Triiodothyronine (FT3) (pg/mL (or pmol/L))[1][2] | 2.3 – 4.2 pg/mL (3.5 – 6.5 pmol/L) — adult, assay-dependent | Low FT3 with raised TSH = hypothyroidism. Low FT3 with normal TSH in an unwell patient = sick euthyroid syndrome (re-check on recovery). Some medications (propranolol, amiodarone, steroids) lower FT3 by blocking T4-to-T3 conversion. | Raised FT3 with suppressed TSH = hyperthyroidism. May rise before FT4 in early Graves' or "T3 toxicosis" — isolated FT3 rise with suppressed TSH is not benign. Exogenous liothyronine (T3) supplementation raises FT3 selectively. |
FT3 patterns
| TSH | FT4 | FT3 | Likely picture |
|---|---|---|---|
| Suppressed | Raised | Raised | Overt hyperthyroidism |
| Suppressed | Normal | Raised | T3 toxicosis or early Graves' |
| Suppressed | Normal | Normal | Subclinical hyperthyroidism |
| Raised | Low | Low or normal | Overt hypothyroidism (T3 preserved until late) |
| Low or normal | Low or normal | Low first | Sick euthyroid syndrome |
| Suppressed | Normal | High (on T3 supplement) | Exogenous liothyronine |
Frequently asked questions
Do I need FT3 if my TSH and FT4 are normal?
Usually no. In a routine thyroid screen, TSH + FT4 is sufficient. FT3 adds value when hyperthyroidism is suspected, when monitoring antithyroid drug therapy, or when on liothyronine (T3) supplementation.
Why is FT3 sometimes raised when FT4 is normal?
This pattern — high FT3 with normal FT4 and suppressed TSH — is called T3 toxicosis. It can be seen in early Graves' disease, autonomous nodules producing predominantly T3, or self-treatment with liothyronine.
My FT3 is low but I feel fine — what now?
An isolated low FT3 with normal TSH and FT4 in someone who feels well usually means non-thyroidal illness (sick euthyroid syndrome) — particularly if you have been unwell, fasting or on certain medications. Re-check after 4–6 weeks.
Should I take T3 supplements if my FT3 is on the low side?
No. Routine T3 (liothyronine) supplementation is not recommended for low-normal FT3 in someone who feels well. The American Thyroid Association and Endocrine Society both reserve T3 use for very specific cases.
Can biotin affect FT3?
Yes — high-dose biotin (in hair-skin-nail supplements) typically gives a falsely raised FT3 and FT4 with a falsely low TSH. Stop 48–72 hours before.
I am on propranolol — will it lower my FT3?
High-dose propranolol can slightly lower FT3 by reducing T4-to-T3 conversion. The effect is small at typical cardiac doses and does not require stopping for the test.
How often is FT3 repeated?
During antithyroid drug treatment for Graves', FT3 (and FT4) is checked every 4–6 weeks initially, then every 2–3 months once stable. In routine hypothyroidism, FT3 monitoring is not needed.
Related Hormones / Endocrine tests
Tests commonly ordered alongside FREE TRIIODOTHYRONINE (Ft3), or that help interpret an unexpected result.
Sources & references
- American Thyroid Association — Thyroid Function Tests · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — T3 Test · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Triiodothyronine · accessed 2026-05-30T00:00:00.000Z
- Indian Thyroid Society — Consensus · accessed 2026-05-30T00:00:00.000Z
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