T3 Test
Also known as: Total T3, Triiodothyronine, Total Triiodothyronine
What is T3?
Total T3 measures all triiodothyronine in the blood. T3 is the more active thyroid hormone and affects energy, metabolism, and heart rate.[MedlinePlus (NIH)]
What This Test Measures
This test measures the total amount of T3 hormone in your blood, including both protein-bound and free T3.[Cleveland Clinic]
Why It's Important
T3 is important for diagnosing hyperthyroidism, especially Graves disease where T3 may be elevated out of proportion to T4.[MedlinePlus (NIH)]
Total T3 is less useful for hypothyroidism since it drops late in the disease.[Cleveland Clinic]
Who Should Get This Test?
Total T3 testing is typically reserved for evaluating hyperthyroidism.
Risk Factors
- Suspected hyperthyroidism with normal T4 (T3 toxicosis)
- Graves disease evaluation and monitoring
- Toxic nodular goiter assessment
- Symptoms of hyperthyroidism: rapid heartbeat, weight loss, tremor, anxiety
- Monitoring patients on antithyroid medications
- Evaluating thyroid storm
- Assessing thyroid function when TSH is suppressed but T4 is normal
Screening Schedule
Not used for routine screening. Ordered when hyperthyroidism is suspected or to monitor treatment.
What Happens During the Test
A blood sample is drawn from a vein in your arm. The lab measures total triiodothyronine levels including both bound and free fractions.
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
Total 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
80 to 200 ng/dL.
Note: Ranges vary by laboratory and age.
Source: MedlinePlus (NIH)Unit Conversion
Convert between ng/dL and nmol/L
| ng/dL | nmol/L |
|---|---|
| 60 | 0.92 |
| 80 | 1.23 |
| 100 | 1.54 |
| 120 | 1.85 |
| 150 | 2.31 |
| 200 | 3.08 |
High Values
High Total T3 indicates hyperthyroidism. It is especially useful when T4 is normal but hyperthyroidism is suspected.[Mayo Clinic]
Low Values
Low Total T3 can indicate severe hypothyroidism or non-thyroidal illness. T3 is the last to fall in hypothyroidism.[Cleveland Clinic]
Test Limitations & Accuracy
Total T3 has important limitations affecting its clinical utility.
- ⚠Affected by binding protein levels (pregnancy, estrogen, liver disease)
- ⚠Drops with any severe non-thyroidal illness (sick euthyroid syndrome)
- ⚠Less useful for hypothyroidism - T3 is maintained until late stages
- ⚠Significant diurnal and day-to-day variation
- ⚠Fasting and caloric restriction lower T3 levels
- ⚠Many medications affect T3 levels
- ⚠Free T3 is preferred when protein binding abnormalities suspected
Alternative & Complementary Tests
T3 testing is usually part of a comprehensive thyroid evaluation.
Preferred when protein binding may affect Total T3
First-line screening test for thyroid dysfunction
Primary confirmatory test for thyroid disease
To calculate Free Thyroxine Index when binding is abnormal
To diagnose autoimmune thyroid disease (Graves, Hashimoto)
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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