Free thyroxine (T4) mass concentration in serum
This test measures the amount of free thyroxine (T4), the active form of thyroid hormone circulating in your blood.
["Free T4 helps assess your thyroid function and whether your thyroid is producing enough hormone."]
Key facts at a glance
- LOINC
- 53349-7
- Short code
- T4 Free DBS-mCnc
- Synonyms
- T4 Free DBS-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 This Test Matters
Your thyroid gland produces hormones that regulate metabolism, energy levels, and many other bodily functions. Free T4 is the form of thyroid hormone that directly enters cells to exert its effects. Measuring it helps doctors diagnose and manage thyroid disorders, such as hypothyroidism or hyperthyroidism, which can affect mood, weight, and energy.
How the Test Works
Blood is drawn from a vein, usually in your arm. The sample is then analyzed in a lab using a sensitive immunoassay method to detect the free T4 hormone in your serum. This test focuses on the unbound (free) portion of T4, which is biologically active.
What to Expect Before the Test
No special preparation is needed for this test. You may eat and drink normally before your blood draw. If you are on thyroid medication, let your healthcare provider know so they can adjust timing if necessary.
What to Expect During the Test
A healthcare professional will insert a small needle into your vein to collect a blood sample. You may feel a slight pinch, but it usually doesn’t cause discomfort. The process takes only a few minutes.
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.
◆Associated findings1
Short name
Free T4
Frequently asked questions
What does a free T4 test tell me about my thyroid health?
How often should I get a free T4 test?
Can I eat or drink anything 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