Free thyroxine (T4) concentration in serum/plasma
This test measures the amount of free (unbound) thyroid hormone T4 in your blood. Free T4 is the active form of thyroid hormone that your body uses.
["Free T4 helps assess your thyroid function and whether your thyroid is producing enough active hormone."]
Key facts at a glance
- LOINC
- 56988-9
- Short code
- T4 Free 1.5h p chal SerPl-mCnc
- Synonyms
- T4 Free 1.5h 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 unexplained weight changes, fatigue, or changes in mood. It may also be used to monitor thyroid hormone levels during treatment for conditions like hypothyroidism or hyperthyroidism.
What Does Free T4 Tell You?
Free T4 levels help determine if your thyroid gland is working properly. While total T4 includes both free and bound hormone, free T4 reflects the biologically active portion. Abnormal levels can indicate thyroid dysfunction, which may require further evaluation or treatment adjustments.
How Is the Test Performed?
A blood sample is taken from a vein, usually in your arm. The sample is then sent to a lab for analysis using a specialized immunoassay method to measure free T4 concentration.
What Are Common Results?
Results are not provided with specific numeric ranges in this summary. Your healthcare provider will interpret the results in relation to your symptoms and other lab tests.
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 is a critical marker for diagnosing and managing thyroid disorders. Low levels may suggest hypothyroidism, while elevated levels could indicate hyperthyroidism. It is often used alongside thyroid-stimulating hormone (TSH) tests to get a clearer picture of thyroid function.
Short name
Free T4
Frequently asked questions
What does a free T4 test measure?
How often should I get a free T4 test?
Can diet or medications 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 16, 2026. · Read our editorial policy