Free thyroxine (T4) concentration in serum/plasma
This test measures the amount of the active form of thyroid hormone (T4) that is not bound to proteins in your blood. It helps assess your thyroid function and whether your thyroid is producing enough hormone.
["A free T4 test helps check if your thyroid is working properly and if you need treatment for low or high hormone levels."]
Key facts at a glance
- LOINC
- 25793-1
- Short code
- T4 Free sp4 p chal SerPl-sCnc
- Synonyms
- T4 Free sp4 p chal SerPl-sCnc
- 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 produces hormones that regulate many bodily functions, including metabolism, energy levels, and mood. Free T4 is the form of thyroid hormone that actually enters your cells to do its job. Measuring it can help doctors diagnose conditions like hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid).
What to Expect Before the Test
No special preparation is usually needed, but your doctor may advise you to avoid certain medications or foods for a few days before the test. You’ll have a small needle inserted into your arm or a blood sample drawn from your finger. The process is quick and typically painless.
What the Results Mean
Results are compared to a reference range provided by your lab. While specific ranges can vary, lower than expected levels may suggest hypothyroidism, while higher levels could indicate hyperthyroidism or other thyroid issues. Your doctor will interpret the results in the context of your overall health.
Common Conditions This Test Helps Diagnose
This test is often used alongside other thyroid tests (like TSH) to diagnose conditions such as Hashimoto’s disease (autoimmune thyroiditis), Graves’ disease, or thyroid nodules. It can also help monitor treatment for thyroid disorders.
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 particularly useful because it reflects the biologically active form of thyroid hormone. Unlike total T4 (which includes bound hormone), free T4 levels can be more sensitive to changes in thyroid function. Doctors may order this test when symptoms like fatigue, weight changes, or hair loss suggest thyroid issues, or when TSH levels are abnormal.
Short name
Free T4
Frequently asked questions
What causes free T4 levels to be checked?
How long does it take to get the results?
Can I take medications 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