Free thyroxine (T4) concentration in serum/plasma
This test measures the amount of active thyroid hormone (T4) that is not bound to proteins in your blood, which can help assess your thyroid function.
["Free T4 helps determine if your thyroid is producing enough active hormone."]
Key facts at a glance
- LOINC
- 56986-3
- Short code
- T4 Free 1h p chal SerPl-mCnc
- Synonyms
- T4 Free 1h p chal SerPl-mCnc
- Category
- chemistry
- Sample
- serum
- Container
- Red-top tube (SST)
- Units
- N/A
- Ref low
- 4.5
- Ref high
- 12
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- CPT
- 84436
- SNOMED
- 302345006
- 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
Why Is This Test Done?
Your healthcare provider may order a free T4 test if you have symptoms of thyroid problems, such as fatigue, weight changes, or unexplained hair loss. It may also be used to monitor thyroid hormone treatment or check for thyroid dysfunction in conditions like hypothyroidism or hyperthyroidism.
What Does a Free T4 Test Tell You?
Free T4 levels can indicate whether your thyroid is functioning normally. While total T4 (including bound hormone) is often measured alongside thyroid-stimulating hormone (TSH), free T4 specifically reflects the biologically active portion of thyroid hormone in your blood.
How Is the Test Performed?
A blood sample is taken from a vein in your arm. You may feel a slight pinch, but the procedure is quick and usually painless. The sample is sent to a lab for analysis using a sensitive immunoassay method to measure free T4 concentration.
What Are Common Results and Next Steps?
Results are not typically provided in absolute numbers on your report, as reference ranges can vary by lab. Your healthcare provider will interpret the results in relation to your symptoms and other test results, such as TSH. Follow-up may include additional thyroid tests or adjustments to medication if needed.
Reference range
Adult: 4.5-12.0 μg/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.
Clinical Significance: Free T4 plays a key role in regulating metabolism, growth, and development. Abnormal levels may signal thyroid disorders, requiring further evaluation to guide treatment, such as hormone replacement or management of underlying conditions.
Short name
Free T4
Frequently asked questions
What does a low or high free T4 level mean?
How often should I get this test?
Can lifestyle changes affect free T4 test results?
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 16, 2026. · Read our editorial policy