Free thyroxine (T4) with high-performance liquid chromatography (HPLC) measurement
This test measures the amount of free thyroxine (T4), a form of thyroid hormone that circulates freely in your blood and is not bound to proteins. It helps assess your thyroid function and whether your thyroid gland is producing enough hormones.
["Free T4 levels help diagnose and monitor thyroid disorders, including hypothyroidism and hyperthyroidism."]
Key facts at a glance
- LOINC
- 58837-6
- Short code
- T4 Free 2h p chal SerPl-mCnc
- Synonyms
- T4 Free 2h 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 thyroid problems, such as fatigue, weight changes, or unexplained mood swings. It may also be used to check thyroid function during pregnancy, before or after thyroid hormone treatment, or to evaluate thyroid nodules.
What to Expect Before the Test
No special preparation is typically needed for this test. You may have a blood sample drawn from a vein in your arm, usually during a routine office visit or at a lab. If you’re taking thyroid medication, let your provider know before the test.
What the Test Results Mean
Free T4 levels alone can sometimes be misleading because they depend on other thyroid-binding proteins in your blood. Your provider will often use this test in combination with thyroid-stimulating hormone (TSH) levels to get a clearer picture of your thyroid health.
Common Conditions Monitored
Free T4 is commonly used to monitor conditions like hypothyroidism (underactive thyroid), hyperthyroidism (overactive thyroid), and thyroiditis (inflammation of the thyroid gland). It can also help detect subclinical 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.
Clinical Significance: Free T4 vs. Total T4
Short name
Free T4 (HPLC)
Frequently asked questions
What causes false results in this test?
How often should I get this test?
Can I eat or drink 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