Thyroxine (T4) Test

What is a thyroxine (T4) test?

Thyroxine is also called T4. T4 is a hormone that your thyroid gland makes. A T4 test measures the level of T4 in a sample of your blood. Too much or too little T4 may be a sign of a thyroid problem.

Your thyroid is a small, butterfly-shaped gland in the front of your neck. It makes hormones that control the way your body uses energy. These hormones affect nearly every organ in your body and control many of your body's most important functions. For example, they affect your breathing, heart rate, weight, digestion, and mood. In children, thyroid hormones affect growth, too.

T4 is the main hormone that your thyroid makes. There are two forms of it in your blood:

• Free T4 is the active form of thyroxine hormone that enters your body tissues where you need it.
• Bound T4 is thyroxine that attaches or binds to certain proteins which prevent it from entering your body tissues. It stays in your bloodstream as a "backup supply" until your tissues need it.

T4 levels can be measured with either a free T4 test or a total T4 test:

• A free T4 test measures the amount of free T4 in your blood. Medical experts believe this test is more accurate than a total T4 test, so it's used more often.
• A total T4 test measures free and bound T4 together.

A T4 test alone can't provide enough information to diagnose thyroid problems. So, it's usually done with a TSH blood test. TSH stands for thyroid stimulating hormone. It's a hormone made by your pituitary gland, a small gland at the base of your brain. TSH tells your thyroid how much hormone to make. Normally, if your T4 levels are too low, your pituitary makes more TSH to make your thyroid work harder. If your T4 levels are too high, your pituitary stops making TSH.

Other names: free thyroxine, free T4, total T4 concentration, Free T4 index (FTI), thyroxine screen, free T4 concentration, thyroxine test by equilibrium dialysis

What is it used for?

A T4 test is usually used 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 may be used to:

• Diagnose:
  • Hypothyroidism, or underactive thyroid, which is when your thyroid gland doesn't make enough thyroid hormone to meet your body's needs.
  • Hyperthyroidism, or overactive thyroid, which is when your thyroid gland makes more thyroid hormone than your body needs.
• Help learn more about:
  • Other thyroid conditions, such as if you have thyroid nodules (growths on your thyroid that aren't cancer) or a goiter (an enlarged thyroid that may make your neck look swollen). Sometimes these conditions can cause high T4 levels.
  • Disorders of your pituitary gland, abnormal T4 levels are usually caused by thyroid problems. But sometimes they are a sign of a pituitary problem that causes too much or too little TSH.
  • Disorders of your hypothalamus, an area of your brain that controls your pituitary gland and other body functions.
• Check a newborn for congenital hypothyroidism, which is hypothyroidism that is present at birth.
• Check your T4 levels, if you are taking thyroid hormone medicine to treat hypothyroidism.

In certain cases, a T4 test may be done as part of a group of thyroid tests called a thyroid panel.

Why do I need a thyroxine (T4) test?

You may need a T4 test if:

• You had abnormal results on a TSH test.
• You have symptoms of hypothyroidism. Not having enough thyroid hormone slows down your body functions. The symptoms can vary from person to person and may include:
  • Fatigue
  • Weight gain
  • Being very sensitive to cold
  • Joint and muscle pain
  • Dry skin
  • Dry, thinning hair
  • Heavy or irregular menstrual periods
  • Fertility problems in women
  • Slow heart rate
  • Depression
  • Constipation
• You have symptoms of hyperthyroidism. Having too much thyroid hormone speeds up your body functions. The symptoms can vary from person to person and may include:
  • Weight loss, even though you may be eating more than usual
  • Rapid or irregular heartbeat
  • Feeling nervous or irritable
  • Trouble sleeping
  • Fatigue
  • Shaky hands, muscle weakness
  • Sweating or being very sensitive to heat
  • Frequent bowel movements (pooping a lot) or diarrhea
  • Goiter
• A member of your family has had thyroid disease. Thyroid disease tends to run in families.
• You're taking thyroid hormone medicine for hypothyroidism. A T4 test may be used to check how well your treatment is working.
• You have symptoms that could be caused by another thyroid condition or a problem with your pituitary gland.

What happens during a thyroxine (T4) test?

A health care 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 into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

Will I need to do anything to prepare for the test?

In most cases, you don't need any special preparations for a T4 blood test. Certain medicines and supplements may affect your test results, so let your provider know what you're taking, but don't stop any medicines without talking with your provider.

If your provider has ordered more tests on your blood sample, you may need to fast (not eat or drink) for several hours before the test. Your provider will let you know if there are any special instructions to follow.

Are there any risks to the test?

There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

What do the results mean?

If you had a free T4 test, your test results may be reported as "free T4." If you had a total T4 test, your results may be reported as "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 usually need to compare them with the results of other thyroid tests. So, it's best to talk with your provider about what your results mean.

In general, T4 results that are lower than normal may be a sign of:

• Hypothyroidism
• Hashimoto's disease, an autoimmune disorder that causes your immune system to attack your thyroid
• Certain stages of thyroiditis (inflammation of the thyroid)
• A pituitary problem (uncommon) or getting too much or too little iodine in your diet
• Congenital hypothyroidism

Low T4 levels may 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:

• Hyperthyroidism
• Graves' disease, an autoimmune disorder that causes your immune system to attack your thyroid
• Certain stages of thyroiditis
• Toxic goiter, an enlarged thyroid that makes too much thyroid hormone
• Toxic thyroid nodule, a lump growing in the thyroid that makes large amounts of thyroid hormone on its own
• Too much iodine, which is found in certain medicines and seaweed
• A noncancerous (not cancer) tumor in your pituitary gland
• Taking too much thyroid hormone medicine to treat hypothyroidism
• A pituitary problem (uncommon)

If your T4 results are abnormal, you may need more testing 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 severe illness and certain medicines, such as birth control pills and steroids.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about a thyroxine (T4) test?

Thyroid disease can develop during pregnancy, but it's not common. If this happens, your provider will treat you, if necessary. After you give birth, you'll usually have your thyroid checked again.

If you have had thyroid disease, be sure to tell your provider if you are pregnant or are thinking of becoming pregnant.