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 without being bound to proteins.
["Free T4 helps assess your thyroid function and whether your thyroid is producing enough active hormone."]
Key facts at a glance
- LOINC
- 3024-7
- Short code
- T4 Free SerPl-mCnc
- Synonyms
- T4 Free 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 to evaluate thyroid function, especially if you have symptoms like fatigue, weight changes, or unexplained issues with metabolism. It’s often used alongside other thyroid tests, such as thyroid-stimulating hormone (TSH).
What Does a Free T4 Test Tell You?
Free T4 levels help determine if your thyroid gland is functioning normally. High levels may indicate hyperthyroidism (overactive thyroid), while low levels can suggest hypothyroidism (underactive thyroid).
How Is the Test Performed?
A blood sample is taken from a vein in your arm. You may feel a slight pinch or discomfort, but it usually doesn’t cause significant pain. The sample is then sent to a lab for analysis.
What Are Common Results?
Results are not provided in absolute numbers here, as reference ranges vary by lab and may not be applicable to individual patients. Your healthcare provider will interpret the results in relation to your symptoms and other test results.
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 crucial for diagnosing and managing thyroid disorders. It reflects the biologically active form of thyroid hormone, which regulates metabolism, growth, and energy levels. Abnormal levels can indicate conditions like Graves’ disease, Hashimoto’s thyroiditis, or thyroid nodules.
Short name
Free T4
Frequently asked questions
What does a free T4 test measure?
How long does it take to get results from a free T4 test?
Can lifestyle factors 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