Free triiodothyronine (T3) in serum/plasma
This test measures the amount of free thyroid hormone T3 in your blood, which is an active form of thyroid hormone produced by your thyroid gland.
["Free T3 helps regulate metabolism and energy levels."]
Key facts at a glance
- LOINC
- 3051-0
- Short code
- T3Free SerPl-mCnc
- Synonyms
- T3Free 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 to check for thyroid function, especially if you have symptoms like fatigue, weight changes, or mood swings. It can also help diagnose or monitor conditions such as hypothyroidism, hyperthyroidism, or thyroid hormone resistance.
What to Expect Before the Test
No special preparation is usually needed. You may be asked to fast for 8–12 hours before the test if your blood is drawn early in the morning. Avoid smoking or caffeine for a few hours before the draw.
How the Test Is Performed
A healthcare professional will draw blood from a vein in your arm using a small needle. You may feel a slight prick or sting, but it usually doesn’t cause much discomfort. The blood sample will be sent to a lab for analysis.
What the Results Mean
Results are interpreted in relation to your overall health and symptoms. Abnormal levels may indicate thyroid dysfunction, but they don’t stand alone—your provider will consider other tests and factors.
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 is a key indicator of thyroid hormone activity. Low levels may suggest hypothyroidism or thyroid suppression, while high levels could indicate hyperthyroidism or overactive thyroid function. It’s often used alongside total T3 and TSH to assess thyroid health.
Short name
Free T3
Frequently asked questions
What causes free T3 levels to be tested?
How long does it take to get results?
Can diet or medications affect free T3 levels?
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