Thyroid Hormone: Free Triiodothyronine (T3) in Serum
This test measures the amount of free triiodothyronine (T3), a key thyroid hormone that helps regulate metabolism, growth, and energy levels in your body.
["Free T3 helps assess thyroid function and identify potential hormone imbalances."]
Key facts at a glance
- LOINC
- 12953-6
- Short code
- T3 sp2 p chal SerPl-mCnc
- Synonyms
- T3 sp2 p chal SerPl-mCnc
- 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 Is This Test Done?
Your healthcare provider may order a free T3 test if you have symptoms like fatigue, weight changes, or unexplained issues with metabolism. It may also be used alongside other thyroid tests to diagnose or monitor conditions such as hypothyroidism, hyperthyroidism, or thyroid dysfunction.
What to Expect Before the Test
No special preparation is needed for this blood test. You may have a small needle inserted into your arm to draw a blood sample. If you’re on thyroid medication, check with your provider first to ensure it’s been taken as usual before the test.
What the Results Mean
Free T3 levels can vary based on individual health, diet, and other factors. While reference ranges exist, they can differ slightly between labs. Your results will be interpreted by your healthcare provider in the context of your overall health and symptoms.
Common Conditions Linked to Free T3 Levels
Abnormal free T3 levels may indicate thyroid disorders, such as underactive (hypothyroidism) or overactive (hyperthyroidism) thyroid function. It can also help identify conditions like thyroiditis or pituitary issues that affect hormone regulation.
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 T3
Frequently asked questions
What causes free T3 levels to be checked?
Can diet or medications affect free T3 levels?
Is fasting required for 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 17, 2026. · Read our editorial policy