Serum Free Triiodothyronine (T3) Concentration
This test measures the amount of free thyroid hormone triiodothyronine (T3) in your blood. Free T3 is the active form of thyroid hormone that helps regulate metabolism, energy levels, and other vital functions.
["Free T3 helps doctors assess thyroid function and identify potential thyroid disorders."]
Key facts at a glance
- LOINC
- 57899-7
- Short code
- T3/T3Reverse SerPl-Rto
- Synonyms
- T3/T3Reverse SerPl-Rto
- 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 This Test Is Done
Your healthcare provider may order a free T3 test if you have symptoms of thyroid problems, such as unexplained weight changes, fatigue, or changes in heart rate. It may also be used to monitor thyroid hormone levels in people with thyroid disease, including those on thyroid replacement therapy.
What to Expect Before the Test
No special preparation is typically needed for this blood test. You may be asked to fast for 8–12 hours before the test if your lab requires it, but this is not always the case. Bring any medications you’re currently taking to your appointment.
What Happens During the Test
A healthcare professional will draw blood from a vein in your arm using a small needle. You may feel a slight pinch, but the procedure usually takes less than five minutes. 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 other thyroid-related tests (such as T4 or thyroid-stimulating hormone). Abnormal levels may suggest thyroid dysfunction, but your doctor will discuss findings with you based on your medical history.
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.
Free T3 is a key indicator of thyroid hormone activity. Low levels may signal hypothyroidism (underactive thyroid), while high levels could indicate hyperthyroidism (overactive thyroid) or other conditions like thyroiditis. It’s often used alongside T4 and TSH to diagnose and manage thyroid disorders.
Short name
Free T3
Frequently asked questions
Will I need to avoid certain foods or medications before the test?
How long will I have to wait for my results?
Can I take this test at home, or do I need to go to a lab?
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