Free Triiodothyronine (T3) in Serum
This test measures the amount of free thyroid hormone T3 in your blood, which is a key indicator of your thyroid function.
["Free T3 helps doctors assess whether your thyroid is producing enough hormone to regulate metabolism and energy levels."]
Key facts at a glance
- LOINC
- 111063-4
- Short code
- T3Free Msmt SerPl
- Synonyms
- T3Free Msmt 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
- 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 like fatigue, weight changes, or unexplained heart issues, or if other thyroid tests (like TSH) suggest an imbalance. It’s also used to monitor thyroid treatment effectiveness or check for thyroid disorders like hyperthyroidism or hypothyroidism.
What Does a Free T3 Test Tell You?
High free T3 levels may indicate hyperthyroidism (overactive thyroid), while low levels can signal hypothyroidism (underactive thyroid) or other conditions affecting thyroid hormone metabolism. It helps doctors diagnose and manage thyroid-related health concerns.
How Is the Test Performed?
A blood sample is taken from a vein in your arm. The sample is sent to a lab where it’s analyzed to measure free T3 levels using a sensitive immunoassay method.
What Are Common Causes of Abnormal Results?
Changes in free T3 can occur due to thyroid diseases (like Graves’ disease or Hashimoto’s thyroiditis), medications (such as amiodarone or steroids), pregnancy, or stress. Other conditions, like kidney or liver disease, may also influence results.
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 does a free T3 test measure?
How often should I get a free T3 test?
Can lifestyle factors (like diet or stress) affect free T3 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