Some of your T4 exists as a 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, it’s 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.
Performing this test may help your doctor gain a better understanding of your thyroid problem.
Your healthcare provider will collect your blood into a tube or vial and send it to a lab for testing.
A T4 test is a blood test that helps your doctor identify problems with your thyroid. It’s a low-risk procedure that’s often used with other blood tests, including a T3 test and a TSH test.
If you’re preparing for a T4 test, make sure to tell your doctor about all medications and supplements you’re taking, any known thyroid conditions, as well as if you’re pregnant. This will ensure the most accurate interpretation of the test results.