Free thyroxine (T4) concentration measured by chemiluminescent immunoassay (CLIA) in serum/plasma
This test measures the amount of free (unbound) thyroxine, a hormone produced by your thyroid gland. Free T4 is the active form of thyroid hormone that circulates in your blood and affects metabolism.
["Free T4 helps regulate your body’s energy, metabolism, and temperature."]
Key facts at a glance
- LOINC
- 27989-3
- Short code
- T4 Free 1h p chal SerPl-aCnc
- Synonyms
- T4 Free 1h p chal SerPl-aCnc
- 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 This Test Might Be Ordered
Your healthcare provider may recommend this test if you have symptoms of thyroid dysfunction, such as unexplained weight changes, fatigue, or changes in heart rate. It may also be used to monitor thyroid hormone levels in people with known thyroid conditions, like hypothyroidism or hyperthyroidism.
What to Expect Before the Test
No special preparation is needed for this test. You may have a blood draw at your doctor’s office or a lab. If you’re on thyroid medication, let your healthcare provider know before the test.
What the Results Mean
Results help determine whether your thyroid is producing too much or too little hormone. Abnormal levels can indicate conditions like hypothyroidism (low levels) or hyperthyroidism (high levels).
Common Conditions Linked to Free T4
Changes in free T4 levels are often associated with thyroid disorders, such as Hashimoto’s disease (autoimmune hypothyroidism), Graves’ disease (autoimmune hyperthyroidism), or thyroid nodules. It can also be checked in people with symptoms of metabolic issues or those taking thyroid replacement medication.
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 thyroid function because it reflects the biologically active form of thyroid hormone. Unlike total T4, which includes protein-bound hormone, free T4 is directly available to tissues, making it a more accurate indicator of thyroid status. Abnormal results may prompt further testing, such as thyroid-stimulating hormone (TSH) levels, to diagnose or manage thyroid disorders.
Short name
Free T4
Frequently asked questions
What does a low free T4 result mean?
What does a high free T4 result mean?
How often should I get 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