T4 (Thyroxine) Test: Free T4 Normal Range and What Results Mean

Nov 7

4 min read

Updated: 3 days ago

Written and medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: November 2026 · How we review

Quick Answer

A T4 (thyroxine) test is a blood test that measures thyroxine, the main hormone the thyroid gland makes. A typical adult free T4 range is about 0.9 to 1.7 ng/dL. High T4 can mean an overactive thyroid, and low T4 can mean an underactive thyroid. Stop biotin supplements 3 to 5 days before, because biotin can skew results.

What the research says

Hidden thyroid problems are common: in the NHANES III survey of the U.S. population, thyroid hormone and antibody levels showed that a large share of people unknowingly had laboratory evidence of thyroid disease, with antibodies more common in women and with age (Hollowell JG et al., J Clin Endocrinol Metab, 2002) [2]
Biotin can distort results for hours to days: in healthy adults taking 5 or 10 mg of biotin a day for a week, thyroid test results were falsely altered on some lab platforms; interference lasted about 8 hours after the 5 mg dose and up to 1 to 2 days after the 10 mg dose (Zhang Y et al., Medicine (Baltimore), 2020) [3]

Key takeaways

T4 is the main hormone made by the thyroid.
Free T4 is usually more accurate than total T4.
Results are best interpreted with TSH.
Adult free T4 is typically about 0.9 to 1.7 ng/dL.
Stop biotin 3 to 5 days before the test.

What is a T4 test?

Your thyroid releases mostly T4, which your liver and kidneys convert into T3, the more active form. Together they control your metabolism, heart rate, digestion, muscles, brain development and bones. Most T4 travels attached to proteins; total T4 measures both bound and free hormone, while free T4 measures only the unbound form that can enter your tissues. A T4 test is often part of thyroid tests or a thyroid panel. Sources: [1]

Why it's done

Follow up an abnormal TSH result.
Diagnose hyperthyroidism or hypothyroidism.
Monitor thyroid hormone treatment such as levothyroxine.
Screen newborns for congenital hypothyroidism.
Evaluate a goiter, thyroid nodules or pituitary problems. Sources: [1]

Normal ranges

Typical free T4 ranges by age: up to 5 years, 6 to 15 years, 16 to 17 years, adults 0.9 to 1.7 ng/dL

Ranges vary slightly between labs.

Age group	Typical free T4 range
Children up to 5 years	0.8–2.8 ng/dL
Children 6–15 years	0.8–2.1 ng/dL
Males 16–17 years	0.8–2.8 ng/dL
Females 16–17 years	0.8–1.5 ng/dL
Adults 18 and older	0.9–1.7 ng/dL

Check the reference range on your own lab report. Sources: [1]

How to prepare

Preparation tips for a T4 test: usually no fasting, stop biotin 3 to 5 days before, list medicines, quick blood draw

A T4 test is a routine blood draw.

Usually no fasting is needed.
Stop supplements that contain biotin, often found in hair, skin and nail products, 3 to 5 days before.
Tell your provider about medicines, birth control and pregnancy.
The blood draw takes less than five minutes, and results usually take 1 to 2 business days. Sources: [1, 3]

Understanding results

TSH and T4 patterns: low TSH with high T4 usually hyperthyroidism, high TSH with low T4 usually hypothyroidism, both low may be a pituitary problem

T4 makes the most sense when read with TSH.

TSH	T4	Usually suggests
Low	High	Hyperthyroidism
Low	Normal	Subclinical hyperthyroidism
High	Low	Primary hypothyroidism
High	Normal	Subclinical hypothyroidism
Low	Low	A possible pituitary problem

High T4 can come from hyperthyroidism, thyroiditis or too much thyroid medicine. Low T4 can come from autoimmune disease, low iodine or some medicines. Pregnancy and estrogen can raise total T4 while free T4 stays normal. Your provider may add T3 or antibody tests for Graves disease or Hashimoto disease. Sources: [1, 2]

Conclusion

A T4 test is a simple blood test that, together with TSH, shows how well your thyroid is working. Telling your provider about biotin and other medicines helps make sure your results are accurate.

Keep learning: read about thyroid tests and hypothyroidism.

Frequently asked questions

What does a T4 test measure?

The amount of thyroxine, the main thyroid hormone, in your blood.

What is the difference between free T4 and total T4?

Free T4 is unbound and active; total T4 includes protein-bound hormone.

What is a normal free T4 level?

About 0.9 to 1.7 ng/dL in adults, depending on the lab.

Do I need to fast?

Usually not.

Why should I stop biotin?

It can make thyroid results look abnormal when they're not.

What does high T4 mean?

Often an overactive thyroid, thyroiditis or too much medicine.

What does low T4 mean?

Often an underactive thyroid.

Why is TSH tested too?

TSH is the best first test, and T4 is clearer when read with it.

Can pregnancy affect T4?

Yes. It can raise total T4 while free T4 stays normal.

How long do results take?

Usually 1 to 2 business days.

References

Cleveland Clinic. T4 (Thyroxine) Test: What It Is, Function, Purpose & Levels. Cleveland Clinic. Last updated October 3, 2022. https://my.clevelandclinic.org/health/diagnostics/24235-thyroxine-t4-test
Hollowell JG, Staehling NW, Flanders WD, et al. Serum TSH, T(4), and thyroid antibodies in the United States population (1988 to 1994): National Health and Nutrition Examination Survey (NHANES III). J Clin Endocrinol Metab. 2002;87(2):489-499. PMID 11836274. https://doi.org/10.1210/jcem.87.2.8182
Zhang Y, Wang R, Dong Y, et al. Assessment of biotin interference in thyroid function tests. Medicine (Baltimore). 2020;99(9):e19232. PMID 32118725. https://doi.org/10.1097/MD.0000000000019232

Medical disclaimer

This article is for general education only and does not replace advice from a qualified healthcare professional. Talk to your doctor about your own diagnosis and treatment, and seek emergency care right away for severe or sudden symptoms.