Serum/plasma free thyroxine (T4) concentration via immunoassay
This test measures the amount of free (unbound) thyroxine, a hormone produced by your thyroid gland, in your blood. Free T4 is the active form of thyroid hormone that circulates freely in your bloodstream.
["Free T4 helps regulate metabolism and energy levels by assessing your thyroid’s active hormone levels."]
Key facts at a glance
- LOINC
- 12931-2
- Short code
- T4 1h p chal SerPl-mCnc
- Synonyms
- T4 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 to evaluate thyroid function, especially if you have symptoms like fatigue, weight changes, or unexplained issues with metabolism. 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. 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. Inform your provider about any medications you’re taking, as some may affect thyroid hormone levels.
What Happens During the Test?
A healthcare professional will draw blood from a vein in your arm. The blood sample is then sent to a lab for analysis using an immunoassay method to measure free T4 levels.
What Are the Results Meaning?
Results are interpreted in relation to your symptoms and other thyroid-related tests. Low free T4 levels may indicate hypothyroidism, while high levels may suggest hyperthyroidism or other conditions. Your provider will discuss the results with you and determine if further testing or treatment is 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.
◆Associated findings1
Short name
Free T4 Level
Frequently asked questions
What Does This Test Measure?
What Conditions Might This Test Be Used To Diagnose?
How Does This Test Compare to Other Thyroid Tests?
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