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 is not bound to proteins and is the form that directly affects your metabolism and energy levels.
["Your doctor may order this test to check thyroid function and rule out conditions like hypothyroidism or hyperthyroidism."]
Key facts at a glance
- LOINC
- 25804-6
- Short code
- T3Free sp2 p chal SerPl-sCnc
- Synonyms
- T3Free sp2 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
Doctors often request a free T4 test when they suspect thyroid issues, such as when you have symptoms like fatigue, weight changes, or unexplained mood swings. It may also be used as part of a routine thyroid function panel or if you have other thyroid-related tests that show abnormal results. Free T4 helps assess whether your thyroid is producing enough active hormone to keep your body functioning properly.
What to Expect Before the Test
No special preparation is needed for this test. You may have a blood sample drawn from a vein in your arm, similar to other blood tests. If you’re on thyroid medication, let your healthcare provider know before the test, as it may affect results.
How the Test Works
The test measures the concentration of free T4 in your blood using a sensitive laboratory method that separates the hormone from proteins and detects its presence. Free T4 is the form of thyroid hormone that actually reaches your cells to regulate metabolism.
What the Results Mean
Results are typically reported in nanograms per deciliter (ng/dL) or picograms per milliliter (pg/mL), but your doctor will interpret them in the context of your overall health and other tests. Low free T4 levels may indicate hypothyroidism, while very high levels could suggest hyperthyroidism or other conditions.
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.
◆Associated findings1
Short name
Free T4
Frequently asked questions
What causes free T4 levels to be tested?
What are the most common symptoms linked to abnormal free T4 levels?
Can free T4 levels change with thyroid medication?
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