Free thyroxine (T₄) concentration in serum/plasma
This test measures the amount of active, unbound thyroid hormone (T₄) in your blood, which helps assess your thyroid function and hormone balance.
["Free T₄ levels help diagnose and monitor thyroid disorders, including hypothyroidism or hyperthyroidism."]
Key facts at a glance
- LOINC
- 25811-1
- Short code
- T3Free sp9 p chal SerPl-sCnc
- Synonyms
- T3Free sp9 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 Matters
Your thyroid gland produces hormones that regulate metabolism, energy, and other vital functions. Free T₄ is the form of thyroid hormone that directly enters your cells. Measuring it helps doctors determine if your thyroid is working properly, even if total T₄ levels appear normal.
How It’s Used in Diagnosis
Free T₄ is often ordered when total T₄ alone doesn’t provide enough information. It’s particularly useful in cases where thyroid antibodies are present (e.g., Hashimoto’s thyroiditis) or when other thyroid tests show conflicting results.
What to Expect Before the Test
No special preparation is needed, but you may be asked to fast for 8–12 hours before your blood draw if your doctor recommends it. Avoid taking thyroid medications (if applicable) for 24 hours before testing unless instructed otherwise.
Common Conditions Monitored
Free T₄ is used to evaluate thyroid dysfunctions like hypothyroidism (low levels) or hyperthyroidism (high levels). It may also help assess thyroid hormone resistance or monitor treatment effectiveness in conditions such as pituitary disorders or thyroid cancer.
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
Will my free T₄ level be affected by thyroid medications?
What if my free T₄ is outside the expected range?
Does stress or illness impact free T₄ levels?
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