Free thyroxine (T4) concentration in serum/plasma
This test measures the amount of free (unbound) thyroxine, a hormone produced by the thyroid gland, in your blood. Free T4 is the active form of thyroid hormone that circulates freely in your bloodstream.
["Free T4 helps assess your thyroid function and whether your body is producing enough active thyroid hormone."]
Key facts at a glance
- LOINC
- 25792-3
- Short code
- T4 Free sp3 p chal SerPl-sCnc
- Synonyms
- T4 Free sp3 p chal SerPl-sCnc
- 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 goiter. It’s often used alongside thyroid-stimulating hormone (TSH) tests to diagnose or monitor thyroid disorders such as hypothyroidism or hyperthyroidism.
What to Expect Before the Test
No special preparation is usually needed, but you may be asked to fast for 8–12 hours before the test if your blood is drawn early in the morning. Avoid smoking or consuming caffeine for at least 30 minutes before the draw.
During the Test
A healthcare professional will draw blood from a vein in your arm. You may feel a slight pinch, but the procedure is quick. The blood sample is then sent to a lab for analysis.
What the Results Mean
Results can vary based on individual health, age, and other factors. Your provider will interpret the results in the context of your overall health and may compare them to your previous test results. Abnormal levels may indicate thyroid dysfunction or other conditions.
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.
◆Associated findings1
Short name
Free T4
Frequently asked questions
How long does it take to get the results of this test?
What are the normal ranges for free T4?
Can I take thyroid medication before this test?
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