The thyroxine (T4) test is essential to assess the function of your thyroid gland, a small gland that is crucial for metabolism and body energy. By measuring T4 levels in the blood, this test helps diagnose conditions such as hypothyroidism and hyperthyroidism, which are essential for your well-being.
Thyroxine is also called T4. T4 is a hormone that your thyroid gland produces. A T4 test measures the level of T4 in a sample of your blood. Too much or too little T4 can be a sign of a thyroid problem.
Your thyroid is a small butterfly-shaped gland located at the front of your neck. It produces hormones that control how your body uses energy. These hormones affect nearly every organ in your body and control many of the essential functions in your body. For example, they affect your breathing, heart rate, weight, digestion, and mood. In children, thyroid hormones also affect growth.
T4 is the main hormone that your thyroid produces. There are two forms of T4 in your blood:
T4 levels can be measured by a free T4 test Or a Total T4 test :
A T4 test alone cannot provide enough information to diagnose thyroid problems. So it is usually done with a TSH blood test. TSH stands for thyroid stimulating hormone. It's a hormone produced by your pituitary gland, a small gland at the base of your brain. TSH tells your thyroid how many hormones to make. Normally, if your T4 levels are too low, your pituitary gland produces more TSH to make your thyroid work harder. If your T4 levels are too high, your pituitary gland stops producing TSH.
Other names: free thyroxine, free T4, total T4 concentration, free T4 index (FTI), thyroxine screening, free T4 concentration, thyroxine balance dialysis test
A T4 test is generally used along with other thyroid tests to help diagnose and monitor thyroid disease and to gather more information about other conditions that may affect your thyroid. It can be used to:
In some cases, a T4 test may be done as part of a group of thyroid tests called a thyroid panel.
You may need a T4 test if:
A health professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected in a test tube or vial. You may feel a slight prick when the needle goes in or out. This usually takes less than five minutes.
In most cases, you do not need special preparations for a T4 blood test. Some medications and supplements may affect your test results, so let your provider know what you're taking, but don't stop any medication without talking to your provider.
If your provider has requested additional tests on your blood sample, you may need to fast (not eat or drink) for several hours prior to the test. Your provider will let you know if there are any special instructions to follow.
There is very little risk in taking a blood test. You may feel a slight pain or bruise where the needle was inserted, but most symptoms go away quickly.
If you have taken a free T4 test, your test results may be reported as “free T4.” If you have taken a total T4 test, your results may be reported as a “free T4 Index (FTI).” FTI is the amount of free T4 in your blood based on a calculation using your total T4 test results.
To fully understand your results, your provider will generally need to compare them with the results of other thyroid tests. So it's best to talk to your provider about what your results mean.
In general, T4 results that are lower than normal may be a sign of:
Low T4 levels can also be caused by treatment for hyperthyroidism or thyroid cancer, including thyroid radiation therapy, radioactive iodine, and surgery to remove all or part of your thyroid.
In general, T4 results that are higher than normal may be a sign of:
If your T4 results are abnormal, you may need additional tests to find the cause. But abnormal T4 levels don't always mean you have a medical condition that needs treatment. Many things can affect your T4 levels, including serious illness and certain medications, such as birth control pills and steroids.
Learn more about lab tests, reference ranges, and understanding results.
Thyroid disease can develop during pregnancy, but it's not common. If this happens, your doctor will treat you, if needed. After you give birth, you will usually have your thyroid checked again.
If you have had thyroid disease, be sure to tell your doctor if you are pregnant or planning to become pregnant.
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