Thyroid hormone levels: Free T3 (triiodothyronine) and Free T4 (thyroxine) in serum
This test measures the active forms of thyroid hormones in your blood, which help regulate metabolism and energy levels.
["Free T3 and T4 help assess thyroid function and rule out conditions like hypothyroidism or hyperthyroidism."]
Key facts at a glance
- LOINC
- 70152-4
- Short code
- T3/T4 SerPl
- Synonyms
- T3/T4 SerPl
- 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
- 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 Are These Tests Done?
Your healthcare provider may order these tests if you have symptoms like unexplained weight changes, fatigue, or changes in heart rate. They may also be used to monitor thyroid hormone levels after treatment for thyroid disorders or during pregnancy.
What Does This Test Measure?
Free T3 and T4 are the hormone forms that circulate freely in your blood and are most directly available to your body’s tissues. Unlike bound forms, these are not attached to proteins and can be more easily measured to evaluate thyroid function.
How Is the Test Performed?
A blood sample is taken from a vein in your arm. The serum is then analyzed to determine the levels of free T3 and free T4 using sensitive laboratory techniques.
What Are the Results Typically Used For?
Results help diagnose thyroid disorders, guide treatment plans, and monitor long-term management of thyroid conditions. They are often interpreted together with other thyroid-related tests.
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.
Clinical Significance: Free T3 and T4 are critical indicators of thyroid health. Low levels may suggest hypothyroidism, while high levels could indicate hyperthyroidism. These tests are essential for early detection and management of thyroid diseases, which can affect metabolism, mood, and overall well-being.
Short name
Free T3 and Free T4
Frequently asked questions
What does a low or high result mean?
How often should I get this test?
Can lifestyle changes affect these 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