Free Thyroxine (T4) in Serum or Plasma, Calculated from Thyroid-Stimulating Hormone (TSH) and Total Thyroxine (T4) Levels
This test measures the amount of free thyroxine (T4) in your blood, calculated based on your TSH and total T4 levels. Free T4 is the active form of thyroid hormone that affects metabolism.
["Understanding your free T4 helps assess thyroid function and identify potential hormonal imbalances."]
Key facts at a glance
- LOINC
- 32215-6
- Short code
- FT4I SerPl Calc-mCnc
- Synonyms
- FT4I SerPl Calc-mCnc
- Category
- chemistry
- Sample
- serum
- Container
- Red-top tube (SST)
- Units
- N/A
- Ref low
- 0.8
- Ref high
- 1.8
- Critical range
- — – 7.8 N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- Method
- calculated
- 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 Is Done
Your healthcare provider may order this test if you have symptoms of thyroid issues, such as fatigue, weight changes, or irregular heartbeats. It can also be used to monitor thyroid conditions like hypothyroidism or hyperthyroidism, especially when TSH alone isn’t sufficient for diagnosis.
How the Test Works
Blood is drawn, and the test calculates free T4 by combining your TSH and total T4 levels. This method helps determine the biologically active portion of thyroid hormone in your bloodstream.
What to Expect Before the Test
No special preparation is usually needed, but you may be asked to fast for 8–12 hours before the blood draw if your test includes fasting components. Avoid smoking or caffeine for 30 minutes before testing.
What to Expect During the Test
A healthcare professional will insert a needle into your arm to draw blood. You may feel a slight pinch, but the procedure usually takes just a few minutes. You can resume normal activities afterward.
Reference range
Adult: 0.8-1.8 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.
This test helps diagnose and manage thyroid disorders by providing a more precise measure of thyroid hormone activity. While TSH is a key indicator, free T4 helps confirm whether your thyroid is producing too much or too little hormone, especially in cases where TSH levels are borderline or abnormal.
Short name
Free T4 Index
Frequently asked questions
What does a free T4 index tell me about my thyroid health?
How often should I get this test?
Can diet or medications affect my free T4 test results?
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