Free thyroxine (T4) concentration in serum/plasma
This test measures the amount of free thyroxine (T4), the active form of thyroid hormone circulating in your blood.
["Free T4 helps regulate metabolism and energy levels by assessing your thyroid’s hormone activity independently of binding proteins."]
Key facts at a glance
- LOINC
- 58839-2
- Short code
- T3Free 3h p chal SerPl-mCnc
- Synonyms
- T3Free 3h p chal SerPl-mCnc
- Category
- chemistry
- Sample
- serum
- Container
- Red-top tube (SST)
- Units
- N/A
- Ref low
- 80
- Ref high
- 200
- Critical range
- 20 N/A – 400 N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- CPT
- 84436
- SNOMED
- 302346007
- Specimen volume
- 5 mL
- Stability
- Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
- Hemolysis max
- 500
- Lipemia max
- 1000
- Icterus max
- 500
- TAT
- 4–24 hours
- Order
- routine
What Does Free T4 Measure?
Free T4 is the portion of thyroid hormone that is not bound to proteins in your blood. Unlike total T4, which includes both free and bound hormone, free T4 reflects the biologically active fraction of thyroid hormone. This test is particularly useful for diagnosing and monitoring thyroid dysfunction, especially when other thyroid tests show conflicting results.
Why Is Free T4 Important?
Measuring free T4 helps doctors evaluate whether your thyroid is producing enough hormone to support normal bodily functions. It can help identify subtle thyroid issues that might not be detected by other tests alone.
How Is This Test Performed?
A blood sample is drawn from a vein, usually in your arm. The sample is then processed to measure the free T4 concentration using a specialized immunoassay method.
What Does a Free T4 Test Tell You?
A free T4 test can indicate whether your thyroid is overactive (hyperthyroidism) or underactive (hypothyroidism). Low levels may suggest hypothyroidism, while high levels may suggest hyperthyroidism or other conditions affecting thyroid hormone metabolism.
Reference range
Adult: 80-200 ng/dLN/A
Take your thyroid medication after the blood draw if applicable. No fasting required.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
Free T4 is critical in diagnosing and managing thyroid disorders because it provides a direct measure of the hormone’s availability to tissues. It helps clinicians assess thyroid function more accurately than total T4 alone, especially in cases of thyroid-binding globulin abnormalities or pregnancy.
Short name
Free T4
Frequently asked questions
What causes abnormal free T4 levels?
How often should I get this test?
Can lifestyle changes affect free T4 levels?
Sources
- 1
- 2Lab Tests OnlineSource
- 3
Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.
This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.
This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.
If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.
Medically reviewed by WellAlly Editorial Team Last reviewed August 17, 2026. · Read our editorial policy