Free T4
Thyroid prohormone thyroxine
Also known as
Definition
Free T4 (fT4, free thyroxine) is the primary secretory product of the thyroid gland and acts as a prohormone, converted to biologically active T3 in peripheral tissues (liver, kidney, muscle) through deiodination. The combination of TSH and fT4 allows differentiation between primary and secondary thyroid disorders. Reference values vary by assay method and laboratory.
| Parameter | Value |
|---|---|
| Unit | ng/dl (pmol/l) |
| Reference Range | 0.7–1.9 ng/dl (9.0–24.5 pmol/l) |
Agree on your personal target range with your doctor.
Conversion: 1 ng/dl × 12.87 = pmol/l. US labs often report ng/dl; international studies use pmol/l. Reference ranges are assay-dependent; the interval of your specific lab is what counts.
↓ What a low value means
Low fT4 with elevated TSH = primary hypothyroidism (most common cause: Hashimoto's thyroiditis, iodine deficiency, resection). Low fT4 with low/normal TSH = secondary hypothyroidism (pituitary insufficiency). Symptoms: fatigue, cold intolerance, weight gain, hair loss, dry skin, depression, constipation, slowed pulse.
↑ What a high value means
Elevated fT4 with suppressed TSH = hyperthyroidism (Graves' disease, toxic adenoma, iodine excess). Elevated fT4 with normal/elevated TSH = TSH-producing pituitary tumor (rare) or thyroid hormone resistance. Elevated fT4 values can also result from assay interference from biotin supplementation or antibodies.
✓ What can influence this value
Free T4 is influenced by thyroid diseases such as hypothyroidism and hyperthyroidism, iodine supply (iodized salt, seafood), selenium status, and very large amounts of raw cruciferous vegetables (glucosinolates can inhibit iodine uptake in iodine deficiency). Levothyroxine therapy acts directly on the value. Preparations containing biotin can distort immunoassay measurements, so biotin intake before the blood draw is relevant for the measurement. Measurement methods differ between laboratories.
When to test
Usually measured together with TSH in thyroid diagnostics. Fasting is not needed. Taking levothyroxine shortly before the blood draw can affect the value, and preparations containing biotin can distort the measurement. When the tablet is taken and how often the value is rechecked is decided by your doctor.
Frequently asked questions
What is the difference between TSH, fT4, and fT3? +
TSH is the pituitary regulatory hormone and rises when the thyroid is underactive. fT4 is the thyroid's main product. fT3 is the active hormone produced from fT4 in peripheral tissues. All three together provide a complete picture of the thyroid axis.
Can biotin skew my fT4 result? +
Yes. High-dose biotin (> 5 mg/day) interferes with immunochemical assays and can make fT4 appear falsely elevated or decreased. Whether biotin is paused before the blood draw is decided by your doctor.
What does it mean if fT4 is normal but TSH is elevated? +
This is subclinical hypothyroidism: the thyroid still produces sufficient fT4 but is stimulated more strongly. Whether and how treatment is given is decided by your doctor.
Free T4
Sources
Last updated: May 28, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.