Triiodothyronine (T3) Tests
A triiodothyronine (T3) test measures the level of T3 in your blood to help diagnose thyroid disease and monitor treatment.
What is a triiodothyronine (T3) test?
A triiodothyronine (T3) test measures the level of T3 in a sample of your blood. Your thyroid, a small, butterfly-shaped gland at the base of your throat, produces T3 and T4 (thyroxine). These hormones act as chemical messengers that control how your body uses energy, affecting your weight, heart, body temperature, muscle strength, and mood. In children, they also affect growth.
There are two forms of T3 in your blood:
- Free T3: The active form that enters your tissues where it is needed.
- Bound T3: Attached to certain proteins which prevent it from entering your tissues. Most of your T3 is bound.
There are two primary ways to measure T3:
- Total T3 test: Measures both bound and free T3 together. Medical experts consider this the more accurate way to measure T3.
- Free T3 test: Only measures free T3.
What is it used for?
A T3 test is used to help:
- Diagnose hyperthyroidism: A condition where the thyroid gland makes too much thyroid hormone.
- Diagnose pituitary disorders: The pituitary gland produces TSH (thyroid stimulating hormone), which tells the thyroid how much T3 and T4 to make. Abnormal T3 levels may indicate a pituitary problem.
- Monitor treatment: Used for thyroid diseases, including hyperthyroidism and hypothyroidism (when the thyroid makes too little hormone).
Why do I need a T3 test?
You may need a T3 test if you have symptoms of hyperthyroidism, such as:
- Anxiety, nervousness, or irritability
- Weight loss despite increased appetite
- Shaky hands or muscle weakness
- Sweating or sensitivity to heat
- Irregular heartbeat (arrhythmia)
- Frequent bowel movements
- Goiter (enlarged thyroid gland)
- Trouble sleeping and fatigue
Additionally, you may need this test if you take thyroid hormone medicine to ensure your dosage is correct, or if you had abnormal results on a TSH or T4 test.
What happens during a T3 test?
A health care professional will take a blood sample from a vein in your arm using a small needle. The process typically takes less than five minutes.
What do the results mean?
Providers compare T3 results with TSH and T4 tests to determine your thyroid health. In general:
- Normal T3 levels: Do not rule out thyroid disease on their own, but if TSH and free T4 levels are also normal, you likely do not have a thyroid condition.
- High T3 levels: May indicate hyperthyroidism. If TSH is low and free T4 is high, the problem is likely in the thyroid gland. If TSH is normal or high and free T4 is high, the problem may be in the pituitary gland.
- Low T3 levels: May indicate hypothyroidism. If TSH is high and free T4 is low, the problem is likely in the thyroid gland. If TSH and free T4 are both low, the problem may be in the pituitary gland.