Free thyroxine (T4) concentration in serum/plasma
This test measures the amount of active (free) thyroid hormone T4 in your blood, which isn’t bound to proteins. It helps assess your thyroid function independently of protein levels.
["Free T4 helps diagnose and monitor thyroid disorders by reflecting your body’s true hormone activity."]
Key facts at a glance
- LOINC
- 25809-5
- Short code
- T3Free sp7 p chal SerPl-sCnc
- Synonyms
- T3Free sp7 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 It’s Done
Your doctor may order a free T4 test if you have symptoms of an underactive (hypothyroidism) or overactive (hyperthyroidism) thyroid, unexplained weight changes, fatigue, or other thyroid-related issues. It’s also used to check thyroid function in pregnancy or when other thyroid tests are inconclusive.
How It Works
A blood sample is taken from a vein in your arm. The lab uses a sensitive immunoassay (or similar method) to detect the free T4 hormone in your serum or plasma, separating it from protein-bound forms.
What to Expect
The procedure is quick and usually painless. You may feel a slight pinch when the needle is inserted, but it’s over in minutes. Results are typically ready within a few days.
What the Results Mean
Free T4 levels alone don’t provide a complete picture—your doctor will consider them alongside other thyroid tests (like TSH) and your symptoms. Abnormal results may prompt further evaluation.
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: Free T4 is critical because it directly reflects thyroid hormone activity in your body. Low free T4 often signals hypothyroidism, while high levels may indicate hyperthyroidism. It’s particularly useful when protein levels (like T4-binding globulin) are unstable or unknown.
Short name
Free T4
Frequently asked questions
What causes a free T4 test to be ordered?
How long does it take to get results?
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 17, 2026. · Read our editorial policy