Free thyroxine (T4) concentration in serum/plasma
This test measures the amount of free thyroxine (T4), a hormone produced by your thyroid gland that plays a key role in regulating metabolism and energy levels.
["Free T4 helps doctors assess thyroid function and identify potential thyroid disorders."]
Key facts at a glance
- LOINC
- 25808-7
- Short code
- T3Free sp6 p chal SerPl-sCnc
- Synonyms
- T3Free sp6 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
What Does Free T4 Measure?
Free T4 is the biologically active form of thyroxine that circulates in your bloodstream without being bound to proteins. Unlike total T4, free T4 levels are not influenced by changes in protein levels, making them a more accurate indicator of thyroid hormone availability in your body.
Why Is Free T4 Important?
Measuring free T4 helps diagnose and monitor conditions like hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid). It also aids in evaluating thyroid hormone resistance or other metabolic issues.
How Is This Test Performed?
A blood sample is drawn from a vein, usually in your arm. The sample is processed to separate serum or plasma, where the free T4 concentration is measured using an immunoassay method to detect and quantify the hormone.
What Are Common Reasons for Testing?
Your healthcare provider may order this test if you have symptoms like fatigue, weight changes, heat intolerance, or cold sensitivity; if you have a known thyroid disorder; or as part of routine thyroid function testing.
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 causes abnormal free T4 levels?
How long does it take to get results?
Can lifestyle or diet affect free T4 test results?
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