Free thyroxine (T4) concentration in serum/plasma
This test measures the amount of free thyroxine (T4), the active form of thyroid hormone circulating in your blood.
["Free T4 helps regulate metabolism and energy levels by assessing your thyroid’s hormone activity independently of protein binding."]
Key facts at a glance
- LOINC
- 25803-8
- Short code
- T3Free sp1 p chal SerPl-sCnc
- Synonyms
- T3Free sp1 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 unbound, biologically active portion of thyroxine in your blood. Unlike total T4, which includes both free and protein-bound forms, free T4 reflects your actual thyroid hormone availability. This is important because protein-bound T4 is not directly available to your body’s tissues.
Why Is Free T4 Important?
Measuring free T4 helps doctors diagnose and manage thyroid disorders, such as hypothyroidism (low hormone levels) or hyperthyroidism (high hormone levels), by providing a clearer picture of your thyroid’s function without interference from binding proteins.
How Is This Test Performed?
A blood sample is drawn from a vein, typically in your arm. The sample is then processed to measure the concentration of free T4 using a sensitive immunoassay method, which detects the hormone’s presence without relying on protein binding.
What Might Affect Results?
Several factors could influence free T4 levels, including medications (such as certain thyroid medications or steroids), pregnancy, liver or kidney disease, and thyroid-related conditions. Recent blood donations or transfusions may also affect results.
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.
Clinical Significance
Short name
Free T4
Frequently asked questions
How long does it take to get results from this test?
Can I take anything before this test?
Will this test be done alongside other thyroid tests?
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