Free thyroxine (T4) concentration in serum/plasma
This test measures the amount of free thyroxine (T4), a form of thyroid hormone that circulates freely in your blood and is not bound to proteins.
["Free T4 helps assess your thyroid function and identify potential thyroid hormone imbalances."]
Key facts at a glance
- LOINC
- 25810-3
- Short code
- T3Free sp8 p chal SerPl-sCnc
- Synonyms
- T3Free sp8 p chal SerPl-sCnc
- Category
- chemistry
- Sample
- serum
- Container
- Red-top tube (SST)
- Units
- N/A
- Ref low
- 80
- Ref high
- 200
- Critical range
- 20 N/A – 400 N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- CPT
- 84436
- SNOMED
- 302346007
- 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 Matters for Your Health
Your thyroid gland produces hormones that regulate metabolism, energy levels, and other vital functions. Free T4 is a key indicator of whether your thyroid is working properly. Unlike total T4, which includes hormone bound to proteins, free T4 reflects the biologically active portion of the hormone that your body can use.
How the Test Works
Blood is drawn from a vein, usually in your arm. The sample is then processed to measure the concentration of free T4 in your serum or plasma using a sensitive laboratory method.
What to Expect Before the Test
No special preparation is typically needed for this test. However, if you are taking thyroid medication, your doctor may advise timing your dose around the test to avoid influencing results.
Possible Results and Interpretation
Results are compared to a reference range that reflects typical values in healthy individuals. Abnormal results may suggest conditions like hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid), though further evaluation may be needed.
Reference range
Adult: 80-200 ng/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 could cause my free T4 levels to be different from what’s expected?
Why might my doctor order this test?
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 17, 2026. · Read our editorial policy