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) |
| Optimal Range (lab2go) | 1.0–1.7 ng/dl (lab2go Optimum Zone) |
Conversion: 1 ng/dl × 12.87 = pmol/l. US labs often report ng/dl; international studies use pmol/l. Reference ranges are assay-dependent — always use your specific lab's interval.
↓ 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.
✓ How to optimize this marker
For primary hypothyroidism: levothyroxine therapy as medically prescribed. Lifestyle support: adequate iodine (iodized salt, seafood), moderate selenium, avoid excessive raw cruciferous vegetables (glucosinolates inhibit iodine uptake in deficiency). Pause biotin supplements at least 48 hours before blood draw (falsifies immunochemical measurement).
When to test
Always with TSH for thyroid diagnostics. No fasting required. Morning collection (8–10 am) recommended. Take levothyroxine after the blood draw. Pause biotin at least 2 days before. Recheck 6–8 weeks after dose change.
Frequently asked questions
What is the difference between TSH, fT4, and fT3? +
TSH is the pituitary regulatory hormone and rises with underfunction. 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 falsify my fT4 result? +
Yes. High-dose biotin (> 5 mg/day) interferes with immunochemical assays and can make fT4 appear falsely elevated or decreased. Pause biotin at least 48 hours before blood draw.
What does it mean if fT4 is normal but TSH is elevated? +
This is subclinical hypothyroidism: the thyroid still produces sufficient fT4 but requires stronger stimulation. Depending on TSH level and symptom severity, treatment may be warranted — this is a medical decision.
Sources
Last Reviewed: May 28, 2026 · sina
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.