Free thyroxine (T4) concentration in serum/plasma
This test measures the amount of active thyroid hormone (T4) that is not bound to proteins in your blood, which can help assess your thyroid function.
["Free T4 helps doctors evaluate thyroid hormone availability and potential thyroid disorders."]
Key facts at a glance
- LOINC
- 83122-2
- Short code
- T4 Free SerPl IA-mCnc
- Synonyms
- T4 Free SerPl IA-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
- Method
- ia
- 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 if you have symptoms of thyroid problems, such as unexplained weight changes, fatigue, or changes in your menstrual cycle. It may also be used to monitor thyroid hormone levels during treatment for conditions like hypothyroidism or hyperthyroidism.
What Does a Free T4 Test Tell You?
Free T4 levels can indicate whether your thyroid is producing enough active hormone. While total T4 (bound + free) is often checked first, free T4 is particularly useful because it reflects the hormone’s biological activity. Abnormal levels may suggest thyroid dysfunction, even if total T4 is within normal ranges.
Sample Preparation and Testing Process
A blood sample is drawn from a vein in your arm. The serum or plasma is then analyzed using a specialized immunoassay method to measure the free (unbound) T4 concentration.
What Are the Results Typically Used For?
Results are often interpreted alongside other thyroid tests, such as thyroid-stimulating hormone (TSH), to diagnose or manage thyroid conditions. Free T4 helps clarify whether changes in TSH are due to thyroid hormone imbalances or other factors.
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 critical because it represents the hormone’s true biological effect. High free T4 levels may indicate hyperthyroidism (overactive thyroid), while low levels can signal hypothyroidism (underactive thyroid). Unlike total T4, which includes protein-bound hormone that may not be active, free T4 directly reflects the hormone’s availability for tissue use.
Short name
Free T4
Frequently asked questions
What does a free T4 test measure?
Why might my doctor order a free T4 test?
What should I expect during or after the 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