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.
["Free T4 helps assess thyroid function and identify potential thyroid disorders."]
Key facts at a glance
- LOINC
- 40295-8
- Short code
- T3Free 2h p chal SerPl-sCnc
- Synonyms
- T3Free 2h 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 This Test Is Done
Your healthcare provider may order this test if you have symptoms of an overactive or underactive thyroid, such as unexplained weight changes, fatigue, or changes in heart rate. It can also be used to monitor thyroid hormone levels during treatment for conditions like hypothyroidism or hyperthyroidism.
How the Test Works
A small blood sample is taken from a vein in your arm. The sample is then analyzed in a lab to measure the concentration of free T4 in your blood. Free T4 is the hormone that circulates freely in your bloodstream and is not bound to proteins.
What to Expect Before the Test
No special preparation is usually needed for this test. However, if you are taking thyroid medication, you may need to stop it temporarily or follow specific instructions from your healthcare provider to ensure accurate results.
What the Results Mean
Results are interpreted by your healthcare provider in relation to your overall health and symptoms. Abnormal levels can indicate thyroid dysfunction, which may require further evaluation or treatment.
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.
Free T4 levels are crucial for diagnosing and managing thyroid disorders. High levels may suggest hyperthyroidism, while low levels may indicate hypothyroidism. Monitoring free T4 helps guide treatment decisions, such as adjusting thyroid hormone replacement therapy.
Short name
Free T4
Frequently asked questions
How long does it take to get the results?
Can I eat or drink before this test?
What are the possible side effects of this blood 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