Free thyroxine (T4) concentration in serum
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 responsible for regulating metabolism, growth, and energy levels.
["Your doctor may order this test to check thyroid function or monitor treatment for thyroid disorders."]
Key facts at a glance
- LOINC
- 25796-4
- Short code
- T4 Free sp7 p chal SerPl-sCnc
- Synonyms
- T4 Free sp7 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
Thyroid hormones, including T4, play a vital role in controlling how your body uses energy. Free T4 is the form that directly influences cells throughout your body. Abnormal levels can indicate conditions like hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid).
How the Test Works
A small blood sample is taken from your arm and sent to a lab. The lab uses an immunoassay method to measure the concentration of free T4 in your serum. This test helps assess whether your thyroid is producing enough active hormone or if there’s an imbalance.
Preparation for the Test
No special preparation is needed for this test. You may have a blood draw at your doctor’s office, clinic, or a lab. If you’re on thyroid medication, let your healthcare provider know so they can advise whether to adjust timing before the test.
What to Expect After the Test
After the blood draw, you can return to your normal activities. Results are typically reviewed by your healthcare provider, who will discuss them with you based on your medical history and symptoms. Results may take several days to weeks to receive, depending on the lab.
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
Free T4 levels can help diagnose or monitor thyroid disorders. Low levels may suggest hypothyroidism, while high levels might indicate hyperthyroidism. Your doctor may also use this test alongside other thyroid tests, such as thyroid-stimulating hormone (TSH), to get a full picture of your thyroid health.
How often should I get a free T4 test? Your doctor will decide how often to check your free T4 levels based on your symptoms, medical history, and other tests. Regular monitoring may be needed if you have thyroid disorders or take thyroid medication.
What are the normal ranges for free T4? Free T4 reference ranges can vary slightly between labs. Your doctor will interpret the results in the context of your overall health and other tests. They may also adjust expectations based on your age, sex, or other health conditions.
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