Free Thyroxine (Free T4)

Also known as: FT4, Thyroxine, Free thyroxine index

Free T4 measures the active, unbound fraction of thyroxine — the main hormone produced by the thyroid gland. It is ordered alongside TSH to confirm thyroid diagnoses and guide treatment decisions.

Specimen

Venous blood (serum separator tube)

Turnaround

4–8 hours

Fasting

Not required

Reference Ranges

Measurement	Reference Range	Unit	Note
Free T4 (adults)	0.8 – 1.8	ng/dL
Free T4 (pregnancy, 1st trimester)	0.8 – 1.5	ng/dL

Reference ranges are typical adult values. Your laboratory may use slightly different intervals — always use your lab's reported reference.

What This Test Measures

The thyroid produces mostly T4 (thyroxine), which is converted to the more active T3 in peripheral tissues. About 99.97% of T4 is bound to carrier proteins (thyroxine-binding globulin, albumin). The free T4 measures only the unbound fraction that can enter cells and exert biological effects. Changes in binding proteins — common in pregnancy, OCP use, or liver disease — affect total T4 without affecting free T4.

When Is This Test Ordered?

Confirming hypothyroidism when TSH is elevated
Assessing hyperthyroidism severity when TSH is suppressed
Distinguishing primary from secondary (pituitary) hypothyroidism
Monitoring thyroid hormone replacement therapy
Pregnancy thyroid screening

When Results Are High

High free T4 with suppressed TSH confirms hyperthyroidism. Causes include Graves' disease (most common), toxic multinodular goitre, and thyroiditis. Very high T4 with tachycardia and fever may signal thyroid storm.

When Results Are Low

Low free T4 with high TSH confirms primary hypothyroidism. Low free T4 with low/normal TSH suggests secondary (central) hypothyroidism — pituitary or hypothalamic dysfunction.

Factors That Can Affect Your Result

Biotin (above 5 mg/day) interferes with many T4 immunoassays — stop biotin 2 days before testing
Heparin (low molecular weight) can displace T4 from proteins, briefly raising free T4
Severe non-thyroidal illness suppresses all thyroid hormones (sick euthyroid syndrome)

Tracking This Test Over Time

When adjusting levothyroxine dose, wait 6–8 weeks before retesting — free T4 takes this long to reach a new steady state. Retesting at 4 weeks will show TSH still catching up and may trigger unnecessary dose changes.

Track your Free T4 in Looms

Related Tests

TSH

TSH is the pituitary gland's signal to the thyroid — it rises when the thyroid is underactive and falls when it is overactive. It is the single most sensitive test for detecting thyroid dysfunction.

Medical disclaimer

This page is for educational purposes only and does not constitute medical advice. Reference ranges vary between laboratories, and results must be interpreted in the context of your symptoms, history, and other tests. Always discuss your results with your healthcare provider. Content reviewed by the Looms medical team, 2026-06-01.

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