Free T3 Test
Also known as: FT3, Free Triiodothyronine
What is Free T3?
Free T3 is the unbound, active form of triiodothyronine. T3 is the more active thyroid hormone, converted from T4 in tissues.[MedlinePlus (NIH)]
What This Test Measures
This test measures the free (unbound) T3 in your blood. T3 is more potent than T4 and affects metabolism, heart rate, and body temperature.[Cleveland Clinic]
Why It's Important
Free T3 is useful for diagnosing hyperthyroidism, especially when T4 is normal.[MedlinePlus (NIH)]
Some patients have normal T4 but elevated T3 (T3 toxicosis).[Cleveland Clinic]
Who Should Get This Test?
Free T3 testing is typically reserved for specific clinical situations.
Risk Factors
- Suspected hyperthyroidism with normal T4 (T3 toxicosis)
- Monitoring patients on T3 therapy (liothyronine or desiccated thyroid)
- Graves disease monitoring
- Assessing thyroid function in patients with low T4 but normal TSH
- Evaluating non-thyroidal illness syndrome (sick euthyroid)
- Suspected conversion disorder (T4 to T3 conversion issues)
Screening Schedule
Not used for routine screening. Ordered for specific clinical indications.
What Happens During the Test
A blood sample is drawn from a vein in your arm. The lab measures unbound triiodothyronine levels.
Duration
5 minutes for the blood draw; results usually available within 1-2 days
Discomfort Level
Minimal - brief pinch from needle insertion
Risks & Side Effects
Free T3 testing through blood draw carries very few risks.
- •Minor pain or bruising at the needle site
- •Slight bleeding
- •Rarely, lightheadedness
- •Very rare risk of infection
Normal Range
2.3 to 4.2 pg/mL.
Note: Ranges vary by laboratory.
Source: Cleveland ClinicUnit Conversion
Convert between pg/mL and pmol/L
| pg/mL | pmol/L |
|---|---|
| 2 | 3.1 |
| 2.5 | 3.9 |
| 3 | 4.6 |
| 3.5 | 5.4 |
| 4 | 6.2 |
| 4.5 | 6.9 |
High Values
High Free T3 indicates hyperthyroidism. It may be elevated even when T4 is normal in some cases.[Mayo Clinic]
Low Values
Low Free T3 can indicate hypothyroidism, severe illness, or malnutrition. T3 drops with non-thyroidal illness (sick euthyroid syndrome).[MedlinePlus (NIH)]
Test Limitations & Accuracy
Free T3 has specific limitations that affect its clinical utility.
- ⚠T3 levels drop with any severe illness (non-thyroidal illness syndrome)
- ⚠Less useful for hypothyroidism diagnosis (T3 maintained until late)
- ⚠Significant day-to-day and hour-to-hour variability
- ⚠Biotin supplements can interfere with certain test methods
- ⚠Should be interpreted with TSH and T4
- ⚠Malnutrition and caloric restriction lower T3
Alternative & Complementary Tests
T3 testing is usually part of a broader thyroid evaluation.
Primary screening test for thyroid dysfunction
Main confirmatory test for thyroid disease
Alternative when assessing overall T3 production
Research/specialized settings for conversion disorders
References
Related Condition Guides
Medical Disclaimer
This information is for educational purposes only and is not intended as medical advice. Always consult with a qualified healthcare provider for interpretation of test results and medical decisions.
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