Free thyroxine (T4) concentration in serum/plasma
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 travels through your bloodstream to reach cells in your body.
["Free T4 helps regulate metabolism, energy levels, and body temperature by assessing your thyroid’s function independently of protein carriers."]
Key facts at a glance
- LOINC
- 111274-7
- Short code
- T4 Free Msmt SerPl
- Synonyms
- T4 Free Msmt SerPl
- 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 conditions such as hypothyroidism or hyperthyroidism.
When Might You Need This Test?
You might need a free T4 test if you have a history of thyroid disorders, take thyroid medication, or experience symptoms that suggest thyroid issues. It can also be part of routine check-ups for individuals at higher risk of thyroid problems.
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 blood draw if your test is part of a larger thyroid panel. Avoid smoking or caffeine for at least 30 minutes before the test to get more accurate results.
What Happens During the Test?
A healthcare professional will draw blood from a vein in your arm. The process is quick and typically causes only mild discomfort. The blood sample is then sent to a lab for analysis.
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
What Are the Normal Results?
How Long Will It Take to Get Results?
Can I Take My Thyroid Medication Before the 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