Free thyroxine (T4) concentration in serum/plasma
This test measures the amount of free (unbound) thyroxine hormone in your blood. Free T4 is the active form of thyroid hormone that your body uses.
["Free T4 helps regulate metabolism and energy levels by assessing your thyroid’s ability to release hormone into circulation."]
Key facts at a glance
- LOINC
- 25806-1
- Short code
- T3Free sp4 p chal SerPl-sCnc
- Synonyms
- T3Free sp4 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 Is This Test Done?
Your healthcare provider may order this test to evaluate thyroid function, diagnose conditions like hypothyroidism or hyperthyroidism, or monitor treatment for thyroid disorders. Free T4 is particularly useful because it reflects the biologically active portion of thyroid hormone, unlike total T4, which includes bound hormone that doesn’t circulate freely.
What to Expect Before the Test
No special preparation is needed, but you may be asked to fast for 8–12 hours before the blood draw if your test is part of a comprehensive thyroid panel. Avoid smoking or caffeine for 30 minutes before the test to ensure accurate results.
How the Test Works
A small sample of blood is taken from a vein in your arm. The blood is then analyzed in a lab using an immunoassay method to measure the concentration of free thyroxine. This test helps determine whether your thyroid is producing enough hormone to maintain normal metabolic function.
What the Results Mean
Results are interpreted in the context of your overall thyroid health, including other thyroid tests like TSH (thyroid-stimulating hormone). Low free T4 levels may indicate hypothyroidism, while high levels could suggest hyperthyroidism or other conditions affecting thyroid hormone metabolism.
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.
Clinical Significance
Short name
Free T4
Frequently asked questions
What Are Common Causes of Abnormal Results?
How Often Should I Get This Test?
Does This Test Interact with Medications?
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