Free thyroxine (T4) concentration in 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.
["Free T4 helps assess thyroid function and identify potential hormone imbalances."]
Key facts at a glance
- LOINC
- 56987-1
- Short code
- T4 Free 40M p chal SerPl-mCnc
- Synonyms
- T4 Free 40M p chal SerPl-mCnc
- 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 to check for thyroid disorders, such as hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid). It’s often used alongside other thyroid tests like TSH (thyroid-stimulating hormone) to get a clearer picture of your thyroid health.
What to Expect Before the Test
No special preparation is usually needed. You may have a small needle inserted into your arm to draw blood. If you’re on thyroid medication, check with your provider first to see if you should stop taking it temporarily.
What the Results Mean
Free T4 levels can vary based on factors like age, sex, and overall health. While reference ranges exist, they can differ slightly between labs. Your results will be compared to your own baseline or to a standard range provided by your healthcare team.
Common Conditions This Test Helps Diagnose
Free T4 helps diagnose conditions like Graves’ disease (hyperthyroidism), Hashimoto’s thyroiditis (hypothyroidism), or thyroid nodules. It can also help monitor thyroid hormone levels in people with known thyroid disorders.
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.
Free T4 is important because it reflects the biologically active form of thyroid hormone. Unlike total T4, which includes bound and free hormone, free T4 is not affected by proteins in the blood, making it a more accurate measure of thyroid function. Low free T4 levels often indicate hypothyroidism, while high levels may suggest hyperthyroidism or other thyroid issues.
Short name
Free T4
Frequently asked questions
What does a free T4 test measure?
Why is free T4 different from total T4?
Can I eat or drink before 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 16, 2026. · Read our editorial policy