What this test measures
Thyroxine (T4) is the principal hormone secreted by the thyroid gland — most of it is bound to carrier proteins (thyroxine-binding globulin, transthyretin, albumin) and is biologically inert. Only the small unbound fraction (Free T4, ~ 0.03%) crosses cell membranes and acts on tissues.
The FT4 assay measures this unbound fraction directly (or, more commonly, by an analog immunoassay). Because it is not affected by changes in carrier proteins, FT4 gives a true picture of thyroid hormone output even when binding proteins are abnormal — pregnancy, oestrogen pills, severe liver disease, nephrotic syndrome.
Why it matters
FT4 is one of the two pillars of thyroid testing (alongside TSH). In a routine outpatient, TSH alone is enough to screen for thyroid disease. FT4 is added when the picture is unusual — discordant TSH, suspected central (pituitary) hypothyroidism, pregnancy, on amiodarone or lithium, or to titrate levothyroxine therapy in a patient who is symptomatic despite a "normal" TSH.
In pregnancy, total T4 rises (because oestrogen raises binding proteins) without true hyperthyroidism — FT4 stays normal and is the right test. In central hypothyroidism, TSH is paradoxically normal or low despite real thyroid hormone deficiency — FT4 is the marker that flags it.
How to prepare
No fasting required. Morning sample preferred for consistency. If on levothyroxine, take it after the blood draw — taking it 1–2 hours before transiently raises FT4. Stop biotin / cosmetic supplements for 48–72 hours before. Tell the lab if you have had recent 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 Thyroxine (FT4) (ng/dL (or pmol/L))[1][2] | 0.8 – 1.8 ng/dL (10 – 23 pmol/L) — adult, assay-dependent | Low FT4 with raised TSH = primary hypothyroidism. Low FT4 with low or inappropriately normal TSH = central hypothyroidism (pituitary or hypothalamic disease) — needs pituitary MRI. Low FT4 with low T3 in acute illness = sick euthyroid syndrome (re-check on recovery). | Raised FT4 with suppressed TSH = primary hyperthyroidism (Graves', toxic nodule, thyroiditis). Raised FT4 with normal/raised TSH = rare — TSH-secreting pituitary adenoma, thyroid hormone resistance, assay interference. Always interpret with TSH together. |
FT4 + TSH — pattern recognition
| TSH | FT4 | Likely picture |
|---|---|---|
| Raised | Low | Overt primary hypothyroidism |
| Raised | Normal | Subclinical hypothyroidism |
| Suppressed | Raised | Overt primary hyperthyroidism |
| Suppressed | Normal | Subclinical hyperthyroidism or T3 toxicosis |
| Low or normal | Low | Central hypothyroidism — image pituitary |
| Normal or raised | Raised | TSH-oma or hormone resistance (rare); biotin interference |
Frequently asked questions
Why is Free T4 sometimes preferred over Total T4?
Total T4 includes hormone bound to carrier proteins, whose levels change in pregnancy, on oestrogen pills, in liver disease and in nephrotic syndrome. Free T4 measures only the biologically active fraction and is unaffected by these changes.
My TSH is normal but FT4 is low — what does that mean?
This pattern (low FT4 with non-raised TSH) suggests central hypothyroidism — a pituitary or hypothalamic problem. It needs a pituitary MRI and full pituitary hormone workup.
Will biotin supplements affect my FT4?
Yes — high-dose biotin (often in cosmetic supplements at 5–10 mg, far above the 30 mcg daily need) interferes with many thyroid assays and typically gives falsely high FT4 / FT3 with falsely low TSH. Stop 48–72 hours before testing.
I am pregnant — what is the normal FT4?
Pregnancy-specific ranges apply. Most labs use the non-pregnant range, but FT4 normally falls slightly in the second and third trimesters. The American Thyroid Association recommends trimester-specific ranges where available.
How often should FT4 be repeated?
On a stable thyroid dose, FT4 and TSH every 6–12 months. During dose titration, every 6–8 weeks until stable. After a thyroidectomy or radioactive iodine, every 4–6 weeks initially.
I take my levothyroxine in the morning — should I take it before the test?
Take it after the blood is drawn. Taking it 1–2 hours before can transiently raise FT4 and give a misleading result.
Can drugs other than thyroxine affect FT4?
Yes — amiodarone, lithium, high-dose steroids, phenytoin, carbamazepine and heparin all affect thyroid function or assay results. Tell your doctor about all medications.
Related Hormones / Endocrine tests
Tests commonly ordered alongside FREE THYROXINE (Ft4), or that help interpret an unexpected result.
Sources & references
- American Thyroid Association — Thyroid Function Tests · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Free T4 Test · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Thyroxine · accessed 2026-05-30T00:00:00.000Z
- Indian Thyroid Society — Consensus · accessed 2026-05-30T00:00:00.000Z
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