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 helps assess your thyroid function and whether your thyroid is producing enough active hormone."]
Key facts at a glance
- LOINC
- 56989-7
- Short code
- T4 Free 20M p chal SerPl-mCnc
- Synonyms
- T4 Free 20M p chal SerPl-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 Is This Test Done?
Your healthcare provider may order a free T4 test if you have symptoms of an underactive (hypothyroid) or overactive (hyperthyroid) thyroid, unexplained weight changes, fatigue, or other thyroid-related concerns. It’s also used to monitor thyroid hormone levels during treatment for thyroid disorders.
What to Expect Before the Test
No special preparation is needed. You may have a small needle inserted into a vein in your arm, or a blood sample may be collected from your finger. If you’re on thyroid medication, check with your provider first to ensure timing aligns with your routine.
What the Results Mean
Free T4 results help determine if your thyroid is functioning normally. While reference ranges vary by lab, abnormal levels may signal thyroid issues that require further evaluation, such as thyroid-stimulating hormone (TSH) testing or additional thyroid function assessments.
Common Conditions This Test Helps Diagnose
Free T4 is often used alongside TSH to diagnose or monitor conditions like hypothyroidism, hyperthyroidism, thyroiditis, or thyroid nodules. It can also help identify subclinical thyroid disorders where TSH is abnormal but symptoms may not yet be apparent.
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.
Clinical Significance
Short name
Free T4
Frequently asked questions
How long does it take to get my results back?
Can I eat or drink before this test?
What if my free T4 level is outside the normal range?
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