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.
["Free T4 helps regulate metabolism and energy levels by assessing your thyroid’s function independently of other proteins in your blood."]
Key facts at a glance
- LOINC
- 25790-7
- Short code
- T4 Free sp1 p chal SerPl-sCnc
- Synonyms
- T4 Free sp1 p chal SerPl-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 if you have symptoms of thyroid issues, such as unexplained weight changes, fatigue, or changes in your menstrual cycle. It’s also used to monitor thyroid hormone levels in people with thyroid disorders like hypothyroidism or hyperthyroidism, or those taking thyroid medication.
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 test if your blood is drawn early in the morning. Avoid smoking or caffeine for at least 30 minutes before your blood draw.
How the Test Is Performed
A healthcare professional will draw blood from a vein in your arm using a small needle. You may feel a slight prick, but it usually doesn’t cause pain. The blood sample is then sent to a lab for analysis.
What the Results Mean
Results help determine if your thyroid is producing enough free T4 to support normal bodily functions. Abnormal results may indicate an underactive (hypothyroidism) or overactive (hyperthyroidism) thyroid gland.
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 diagnosing and managing thyroid disorders. While total T4 includes both free and bound hormone, free T4 reflects the biologically active portion that directly influences metabolism. It’s often used alongside thyroid-stimulating hormone (TSH) to provide a more complete picture of thyroid function.
Short name
Free T4
Frequently asked questions
What does a low or high free T4 result mean?
How often should I get this test?
Can lifestyle changes affect my free T4 levels?
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