What this test measures
A Free TFT measures Free T3, Free T4 and TSH. The "free" part is what matters: most circulating thyroid hormone is bound to carrier proteins (TBG, transthyretin, albumin) and biologically inert. Only the free fraction crosses cell membranes and acts on tissues.
Free T4 is the principal storage form and a stable indicator of thyroid output. Free T3 is the more biologically active hormone (3–4× the potency of T4) and is produced mainly by peripheral conversion from T4. TSH is the pituitary signal that drives thyroid production. Measuring all three gives you both the thyroid output and the pituitary feedback loop in one panel — unaffected by changes in binding proteins.
Why it matters
Free TFT is the preferred thyroid panel whenever binding proteins are likely to be abnormal — pregnancy (binding proteins rise sharply with oestrogen), women on oestrogen-containing contraceptive pills, severe liver disease, nephrotic syndrome, or rare congenital binding-protein abnormalities. In those situations the Total panel can be misleading: total T4 can look high in pregnancy without any true hyperthyroidism, while free T4 stays normal.
It is also useful when you need a precise picture of thyroid output — for example, fine-tuning levothyroxine in someone with persistent symptoms despite a "normal" TSH, or working up unusual patterns (low TSH with normal total T4, central hypothyroidism, T3 toxicosis).
How to prepare
No fasting required. Best done in the morning for consistency. If you are on levothyroxine, take it after the blood draw. Stop biotin / cosmetic supplements for 48–72 hours before. Continue your other medications unless your doctor instructs otherwise.
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] | 2.3 – 4.2 pg/mL (3.5 – 6.5 pmol/L) | Low FT3 with raised TSH suggests hypothyroidism. Low FT3 with normal TSH is often sick euthyroid syndrome (FT3 falls first in acute illness). Some medications (propranolol, amiodarone, high-dose steroids) reduce T4 → T3 conversion. | Raised FT3 with suppressed TSH suggests hyperthyroidism. May rise before FT4 in early Graves' disease or "T3 toxicosis"; if FT3 is high but FT4 is normal with suppressed TSH, do not dismiss — workup is still needed. |
| Free Thyroxine (FT4) (ng/dL (or pmol/L))[1] | 0.8 – 1.8 ng/dL (10 – 23 pmol/L) | Low FT4 with raised TSH = primary hypothyroidism. Low FT4 with normal / inappropriately low TSH = central (pituitary) hypothyroidism — uncommon but important to spot, as it needs different management and often pituitary imaging. | Raised FT4 with suppressed TSH = primary hyperthyroidism. Raised FT4 with normal or raised TSH = rare conditions (TSH-secreting pituitary adenoma, thyroid hormone resistance) needing endocrinology referral. |
| TSH (3rd Generation) (mIU/L) | 0.4 – 4.0 general adult · 0.1 – 2.5 1st trimester pregnancy · 0.2 – 3.0 2nd / 3rd trimester | See "Suppressed TSH" in TSH page — suggests hyperthyroidism, over-replacement, or rarely central disease. | Suggests primary hypothyroidism. Subclinical (4–10) is mild and often re-checked; overt (>10) usually needs treatment. |
Free TFT patterns and what they suggest
| Pattern | TSH | FT4 | FT3 | Likely cause |
|---|---|---|---|---|
| Primary hypothyroidism | ↑ | ↓ | ↓ or normal | Hashimoto's, iodine deficiency, post-surgery, drugs |
| Subclinical hypothyroidism | ↑ (4–10) | Normal | Normal | Mild / early thyroid dysfunction |
| Primary hyperthyroidism | ↓ | ↑ | ↑ | Graves' disease, toxic nodule(s), thyroiditis |
| T3 toxicosis | ↓ | Normal | ↑ | Early Graves', autonomous nodule producing T3 |
| Central hypothyroidism | Low / normal (inappropriate) | ↓ | ↓ or normal | Pituitary / hypothalamic disease — needs imaging |
| Pregnancy (normal) | Low–normal (1st trim) | Normal range | Normal range | Use trimester-specific ranges |
| Sick euthyroid | Low / normal | Normal / low | Low | Acute illness — repeat when recovered |
Frequently asked questions
Why would a doctor pick Free TFT over the Total TFT?
Free TFT is preferred when binding-protein levels are likely off — pregnancy, oestrogen pills, severe liver disease, nephrotic syndrome — because Total T4 / T3 can look misleadingly high or low in those situations. It is also used when fine-tuning levothyroxine, or when working up unusual patterns where TSH and Total T4 disagree.
Do I need to fast?
No. The Free TFT does not require fasting. Morning testing is preferred for consistency.
I am pregnant — which thyroid panel should I do?
In pregnancy the Free TFT is preferred because the Total values rise with binding proteins. Trimester-specific TSH targets apply: 0.1–2.5 in the 1st trimester, 0.2–3.0 in the 2nd and 3rd. Anti-TPO antibodies are also worth checking once if not previously done.
My TSH is normal but Free T4 is low — what does this mean?
A low Free T4 with non-raised TSH is unusual and can flag central hypothyroidism (a pituitary or hypothalamic problem), sick euthyroid syndrome during acute illness, or assay interference. Endocrinology review is sensible — pituitary imaging may be needed.
My Free T3 is high but Free T4 is normal and TSH is suppressed — am I hyperthyroid?
This is called "T3 toxicosis" and is a real form of hyperthyroidism. It often represents early Graves' disease or a hyperfunctioning ("hot") nodule producing predominantly T3. It needs the same evaluation as other forms of hyperthyroidism.
Can biotin affect Free TFT results?
Yes — biotin can interfere with many thyroid hormone assays, typically producing falsely low TSH with falsely high Free T4 / T3 (mimicking hyperthyroidism). Stop biotin / cosmetic supplements for 48–72 hours before the test.
How often should I get a Free TFT?
For most adults a routine TSH alone is enough as a screen, with Free T4 added if TSH is abnormal. Full Free TFT every 6–12 months is standard while a levothyroxine dose is being optimised, in pregnancy planning, and during pregnancy.
Should I take my levothyroxine before or after the test?
After. Taking levothyroxine 1–2 hours before the test can transiently raise Free T4 readings. Take it after the blood sample is drawn.
Related Hormones / Endocrine tests
Tests commonly ordered alongside FT3, FT4, TSH (FREE TFT), or that help interpret an unexpected result.
Sources & references
- American Thyroid Association — Thyroid Function Tests · accessed 2026-05-29T00:00:00.000Z
- NIH MedlinePlus — Free T4 Test · accessed 2026-05-29T00:00:00.000Z
- Endocrine Society Clinical Practice Guidelines · accessed 2026-05-29T00:00:00.000Z
- Indian Thyroid Society — Consensus · accessed 2026-05-29T00:00:00.000Z
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