A T3 (triiodothyronine) test is a blood test that helps diagnose thyroid conditions, specifically hyperthyroidism. Healthcare providers typically order this test alongside other thyroid function tests.
Your thyroid is a small, butterfly-shaped gland located at the front of your neck under your skin. It’s a part of your endocrine system. Triiodothyronine, also known as T3, is one of the two main thyroid hormones. Thyroxine, or T4, is the other hormone.
Healthcare providers test T3 levels using blood tests. Triiodothyronine comes in two forms:
Because of this, there are a few different tests that measure T3 levels. A blood test that measures both free T3 and bound T3 is called a total T3 test. A different blood test measures just free T3 levels. The tests for free T3 are generally less accurate than for total T3.
Healthcare providers often order additional tests to assess thyroid function alongside a T3 test, including a T4 (thyroxine) test and a TSH (thyroid-stimulating hormone) test.
Other names for a T3 test include:
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Triiodothyronine, also known as T3, is one of the two main hormones your thyroid gland releases into your bloodstream. Your thyroid also produces thyroxine, also known as T4 and tetraiodothyronine. T4 and T3 work together and are commonly referred to as “thyroid hormone.”
Most of the T3 (approximately 80%) in your blood is from your body’s conversion of T4 into T3 outside of your thyroid gland. The rest of the T3 in your bloodstream is produced by your thyroid gland.
Hormones are chemicals that coordinate different functions in your body by carrying messages through your blood to your organs, muscles and other tissues. These signals tell your body what to do and when to do it.
T3 is the active form of thyroid hormone, meaning it impacts cells in your body, whereas T4 is the inactive form of thyroid hormone. Your liver and kidneys convert most of the T4 your thyroid releases into T3.
Together, T4 and T3 play vital roles in regulating your body’s:
Healthcare providers most often order T3 tests to help diagnose hyperthyroidism, a condition in which your thyroid makes too much thyroid hormone, or to determine the severity of hyperthyroidism.
Your provider may order regularT3 tests to monitor your T3 levels if you’re taking thyroid hormone replacement therapy (medication) for a thyroid condition.
A healthcare provider called a phlebotomist usually performs blood draws, including those for a T3 blood test, but any healthcare provider who is trained in drawing blood can perform this task. They then send the samples to a lab where a medical laboratory scientist prepares the samples and performs the tests on machines known as analyzers.
You usually don’t need to do anything special for a T3 blood test. Depending on the reason for the test, you may need to stop taking certain medications or supplements. In any case, your healthcare provider will give you specific instructions.
You can expect to experience the following during a blood test, or blood draw:
The entire procedure usually takes less than five minutes.
After a healthcare provider has collected your blood sample, they’ll send it to a laboratory for testing. Once the test results are back, your provider will share the results with you.
Blood tests are a very common and essential part of medical testing and screening. There’s very little risk to having a T3 blood test. You may have slight tenderness or a bruise at the site of the blood draw, but this usually resolves quickly.
In most cases, you should have your T4 test results within one or two business days, though it could take longer.
Blood test reports, including T3 test reports, usually provide the following information:
Normal value ranges for any lab test, including T3 (triiodothyronine) tests, may vary slightly among different laboratories. Be sure to check your lab report’s reference range on your results. If you have any questions about your results, ask your healthcare provider.
Normal T3 level ranges vary based on age. In general, normal ranges for T3 for healthy people include:
Providers don’t usually order free T3 tests because they’re not as reliable, but it is possible to test these levels. In general, normal ranges of free T3 for healthy people include:
Higher-than-normal T3 levels typically indicate hyperthyroidism (overactive thyroid). Hyperthyroidism has several causes, including Graves’ disease (an autoimmune condition), thyroid nodules and thyroiditis (inflammation of your thyroid gland).
Hyperthyroidism speeds up your metabolism, which can be dangerous to your health. Some symptoms of hyperthyroidism include:
If you’re experiencing symptoms of hyperthyroidism, it’s important to contact your healthcare provider.
If you’ve already been diagnosed with hyperthyroidism, T3 tests can help determine how severe it is. In general, the more elevated your T3 levels, the more severe the hyperthyroidism is.
Lower-than-normal T3 levels may indicate you have hypothyroidism (underactive thyroid). However, healthcare providers don’t typically rely on T3 tests to diagnose hypothyroidism because it’s usually the last of the thyroid function tests to come back abnormal.
In addition, some people can have severe hypothyroidism with a high TSH level and a low free T4 level but have a normal T3 level.
Lower-than-normal T3 levels can also be due to medications like steroids and amiodarone (arrhythmia medication) and severe illness. These factors can decrease the amount of T4 (inactive hormone) your body converts into T3 (active hormone), resulting in a lower level of T3.
Total T3 test results are usually accurate. However, certain factors may interfere with the results, including certain medications or supplements and pregnancy. Your healthcare provider will consider these factors when interpreting your results.
A note from Cleveland Clinic
Seeing an abnormal test result can be stressful. Know that thyroid conditions are somewhat common and treatable. Your healthcare provider will let you know if you need to undergo further tests to determine the cause of the abnormal T3 level. Don’t be afraid to ask your provider questions. They’re there to help you.
Last reviewed by a Cleveland Clinic medical professional on 02/14/2022.
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