Thyroid Hormone, Free T4 (Serum)
This test measures the amount of free thyroxine (T4), a hormone produced by your thyroid gland that plays a key role in regulating metabolism, growth, and energy levels.
["Free T4 helps assess thyroid function and identify potential hormonal imbalances."]
Key facts at a glance
- LOINC
- 12959-3
- Short code
- T3 sp8 p chal SerPl-mCnc
- Synonyms
- T3 sp8 p chal SerPl-mCnc
- 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 Might Be Ordered
Your healthcare provider may recommend this test if you have symptoms like fatigue, weight changes, heat intolerance, or cold sensitivity, or if they suspect thyroid disorders such as hypothyroidism or hyperthyroidism. It may also be used during pregnancy or to monitor thyroid treatment.
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 serum/plasma is collected. Avoid taking thyroid medications for 24 hours before testing if instructed by your provider.
How the Test Is Performed
A blood sample is drawn from a vein in your arm by a healthcare professional. The blood is sent to a lab for analysis using an immunoassay method to measure free T4 levels.
What the Results Mean
Results are typically reported in nanograms per deciliter (ng/dL) or picograms per milliliter (pg/mL). While reference ranges vary by lab, lower than expected levels may indicate hypothyroidism, and higher levels may suggest hyperthyroidism. Your provider will interpret results in the context of your symptoms and medical history.
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 free T4 levels to be checked?
What might affect my free T4 test results?
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