Free thyroxine (T4) concentration in serum/plasma
This test measures the amount of free thyroxine (T4), the active form of thyroid hormone circulating in your blood.
["Free T4 helps regulate metabolism and energy levels by assessing how well your thyroid is functioning independently of other thyroid-binding proteins."]
Key facts at a glance
- LOINC
- 53350-5
- Short code
- T4 Free DBS-sCnc
- Synonyms
- T4 Free DBS-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 issues with metabolism. It’s often used alongside thyroid-stimulating hormone (TSH) tests to diagnose or monitor conditions like hypothyroidism or hyperthyroidism.
How Is the Sample Collected?
A small amount of blood is drawn from a vein, usually in your arm. The blood sample is sent to a lab for analysis. You may feel a slight pinch when the needle is inserted, but the procedure usually takes less than five minutes.
What to Expect Before or After the Test
No special preparation is needed for this test. You can eat and drink normally before the blood draw. Afterward, you may experience mild bruising or soreness at the injection site, which should resolve quickly. If you’re on blood-thinning medication, let your provider know before the test.
What the Results Mean
Free T4 results help determine if your thyroid hormone levels are within a healthy range. Abnormal results may indicate thyroid dysfunction, which can affect energy, mood, and metabolism. Your provider will interpret the results in the context of your overall health and other tests.
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 Common Causes of Abnormal Results?
How Often Should I Get This Test?
Can I Take Anything 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