Free triiodothyronine (T3) in serum/plasma
This test measures the amount of free (unbound) thyroid hormone T3 in your blood. Free T3 is an active form of thyroid hormone that helps regulate metabolism.
["Free T3 helps assess your thyroid function and whether your thyroid is producing enough active hormone."]
Key facts at a glance
- LOINC
- 83127-1
- Short code
- T3Free SerPl IA-mCnc
- Synonyms
- T3Free SerPl IA-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
- Method
- ia
- 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 doctor may order a free T3 test if you have symptoms of thyroid problems, such as fatigue, weight changes, or unexplained heart issues. It can also be used to monitor thyroid hormone levels when other tests like TSH (thyroid-stimulating hormone) are inconclusive.
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 blood is drawn early in the morning, but this depends on your doctor’s instructions. Avoid smoking or strenuous activity 30 minutes before the draw.
What Happens During the Test?
A healthcare provider will draw blood from a vein in your arm. You may feel a slight pinch, but the procedure is quick. The blood sample is then sent to a lab for analysis using an immunoassay method to measure free T3 levels.
What Are the Results?
Results are typically reported in picograms per deciliter (pg/dL) or similar units. Your doctor will interpret the results in relation to your symptoms and other lab tests, such as TSH.
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 important because it reflects the biologically active portion of thyroid hormone in your blood. Low free T3 levels may indicate hypothyroidism (underactive thyroid), while high levels could suggest hyperthyroidism (overactive thyroid) or other thyroid dysfunctions. It can also help diagnose or monitor conditions like thyroiditis or pituitary disorders.
Short name
Free T3
Frequently asked questions
What does a free T3 test measure?
Why might my doctor order a free T3 test?
Can I eat or drink before a free T3 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