What this test measures
The Thyroid Function Profile (Free) measures three values in a single tube of blood:
• Free T3 (FT3) — the biologically active triiodothyronine, the more potent thyroid hormone. • Free T4 (FT4) — the biologically active thyroxine, the principal storage form of thyroid hormone. • TSH (3rd generation) — the pituitary signal that drives thyroid output.
Unlike total T3/T4, the free fractions are not affected by changes in carrier proteins — so this panel is preferred whenever binding proteins are likely abnormal.
Why it matters
The Free TFT is the preferred thyroid panel in pregnancy (binding proteins rise sharply with oestrogen), in women on oestrogen-containing pills or HRT, in severe liver disease, nephrotic syndrome, and to fine-tune thyroid hormone replacement in patients who remain symptomatic despite a "normal" TSH.
It is also useful in unusual patterns — central (pituitary) hypothyroidism (where TSH is inappropriately normal but FT4 is low), suspected T3 toxicosis, and when monitoring antithyroid drug treatment in Graves' disease. For most uncomplicated outpatient thyroid screening, a TSH alone is adequate — but every Indian outpatient clinic now uses the Free TFT bundle as the working "thyroid full panel".
How to prepare
No fasting required. Best done in the morning for consistency. If on levothyroxine, take the dose after the blood draw. Stop biotin / cosmetic supplements for 48–72 hours before. Continue other medications. Tell the lab if you have recently had iodine-contrast imaging.
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)[1] | 2.3 – 4.2 pg/mL (3.5 – 6.5 pmol/L) | Low FT3 with raised TSH = hypothyroidism. Low FT3 with normal TSH in an unwell patient = sick euthyroid syndrome. | Raised FT3 with suppressed TSH = hyperthyroidism. May rise before FT4 in early Graves' or T3 toxicosis. |
| Free Thyroxine (FT4) (ng/dL)[1] | 0.8 – 1.8 ng/dL (10 – 23 pmol/L) | Low FT4 with raised TSH = primary hypothyroidism. Low FT4 with low / inappropriately normal TSH = central hypothyroidism — needs pituitary imaging. | Raised FT4 with suppressed TSH = primary hyperthyroidism. Raised FT4 with non-suppressed TSH (rare) = TSH-secreting pituitary adenoma or hormone resistance. |
| TSH (3rd Generation) (mIU/L)[1] | 0.4 – 4.0 (general adult); pregnancy-specific in 1st trimester | Suppressed TSH suggests overt or subclinical hyperthyroidism, over-replacement, or biotin interference. | Raised TSH suggests primary hypothyroidism — higher values more likely to need treatment. |
Free TFT — common patterns
| Pattern | TSH | FT4 | FT3 | Likely cause |
|---|---|---|---|---|
| Overt hypothyroidism | ↑ | ↓ | ↓ or normal | Hashimoto's, post-RAI / surgery |
| Subclinical hypothyroidism | ↑ (4–10) | Normal | Normal | Early or mild Hashimoto's |
| Overt hyperthyroidism | ↓ | ↑ | ↑ | Graves', toxic nodule, thyroiditis |
| T3 toxicosis | ↓ | Normal | ↑ | Early Graves' or autonomous nodule |
| Central hypothyroidism | ↓ or normal | ↓ | ↓ | Pituitary or hypothalamic disease |
| Sick euthyroid | ↓ or normal | ↓ or normal | ↓ first | Acute illness — re-check on recovery |
Frequently asked questions
Why is the Free TFT used in pregnancy?
Pregnancy raises binding proteins, which lifts the total T3 and T4 but leaves the free fractions in their normal physiological range. The Free TFT gives a true measurement of thyroid hormone activity in pregnancy.
What's the difference between Free TFT and Total TFT?
Total TFT measures all the thyroid hormone (bound + free); Free TFT measures only the biologically active free fraction. In most adults Total is sufficient; Free is needed when binding proteins are likely abnormal.
Should I stop my biotin supplement before the test?
Yes — high-dose biotin (often in hair-skin-nail supplements) interferes with many thyroid assays and can give falsely high FT4/FT3 and falsely low TSH. Stop 48–72 hours before.
My TSH is normal but FT4 is low — what does that mean?
This pattern (low FT4 with non-raised TSH) needs careful interpretation. Possibilities include central hypothyroidism (pituitary or hypothalamic problem — needs pituitary imaging) or sick euthyroid syndrome. A specialist review is helpful.
How often should the Free TFT be repeated?
On a stable thyroid replacement dose, every 6–12 months. During dose titration, every 6–8 weeks. Annually in pregnancy planning and at each antenatal visit during pregnancy.
Should I take my levothyroxine on the test day?
Take it after the blood is drawn. Taking it 1–2 hours before can transiently raise FT4 and cause misinterpretation.
Why does the lab use "3rd generation TSH"?
Third-generation TSH assays measure down to 0.01 mIU/L, allowing detection of fully suppressed TSH (in hyperthyroidism or over-replacement). Older assays could not distinguish "low" from "very low".
Related Hormones / Endocrine tests
Tests commonly ordered alongside Thyroid Function Profile (TFT), Free, or that help interpret an unexpected result.
Sources & references
- American Thyroid Association — Thyroid Function Tests · accessed 2026-05-30T00:00:00.000Z
- Endocrine Society Clinical Practice Guidelines · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Thyroid Tests · accessed 2026-05-30T00:00:00.000Z
- Indian Thyroid Society — Consensus · accessed 2026-05-30T00:00:00.000Z
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