Thyroxine (T4)

Your thyroid produces a hormone called thyroxine, which is known as T4. This hormone plays a role in several of your body’s functions, including growth and metabolism.

Some of your T4 exists as free T4. This means that it hasn’t bonded to protein in your blood. This is the type available for use by your body and tissues. However, most of the T4 in your bloodstream is bonded to protein.

Because T4 exists in two forms in your body, there are two kinds of T4 tests: a total T4 test and a free T4 test.

A total T4 test measures the T4 that has bonded to protein along with any free T4. A free T4 test measures only the free T4 in your blood. Because free T4 is what’s available to your body for use, a free T4 test is often preferred over a total T4 test.

Your doctor may order a T4 test if a thyroid-stimulating hormone (TSH) test has come back with abnormal results. A T4 test will help your doctor determine what type of problem is affecting your thyroid.

A  number of drugs can interfere with your thyroxine levels, especially total T4, so it’s important to tell your doctor what medications you’re taking. You may need to stop taking certain drugs temporarily before the test to help ensure accurate results. (

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