What is a Thyroxine (T4) Test?

A thyroxine test is a blood test that helps diagnose thyroid conditions. The thyroid is a small, butterfly-shaped gland at the base of your throat. Your thyroid makes hormones that control the way your body uses energy. These hormones affect your weight, heart, body temperature, muscle strength, and even your mood. In children, thyroid hormones affect growth, too.

Thyroxine, also known as T4, is a type of thyroid hormone. A T4 test measures the level of T4 in your blood. Too much or too little T4 can be a sign of thyroid disease.

There are two forms of T4 in your blood:

  • Free T4 is the active form of thyroxine hormone that enters your tissues where it’s needed.
  • Bound T4 is thyroxine that attaches or binds to certain proteins which prevent it from entering your tissues.

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

  • A free T4 test directly measures the amount of free T4 in your blood. Medical experts believe this test provides more accurate information than a total T4 test, so it’s used more often.
  • A total T4 test measures free and bound T4 together. A calculation may be done to figure out how much T4 is free. This is a less accurate way to check how well your thyroid is working.

Other names: free thyroxine, free T4, total T4 concentration, T4 index, thyroxine screen, free T4 concentration, thyroxine test by equilibrium dialysis

What is it used for?

A T4 test is used to check how well your thyroid is working. It’s also used to help diagnose and monitor thyroid disease and conditions that may affect your thyroid, including:

  • 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
  • Disorders of the pituitary gland, which is a gland at the base of your brain that makes TSH (thyroid stimulating hormone). TSH controls how much T4 your thyroid gland makes. If the results from a TSH test aren’t normal, a T4 test can help diagnose whether the problem is with your thyroid or your pituitary gland.

In certain cases, a T4 test may be done as part of a group of tests called a thyroid panel. A thyroid panel includes a test for TSH as well as other thyroid tests.

Why do I need a thyroxine test?

You may need a T4 test if your provider thinks you may have a thyroid problem. Usually, a TSH test is done first. If your TSH results aren’t normal, a T4 test may be done to learn more.

Thyroid problems tend to run in families, so your provider may order a T4 test if a member of your family has had thyroid disease. You may also need a T4 test if you have symptoms of having too much or too little thyroid hormone.

Symptoms of hypothyroidism (too little thyroid hormone), may include:

  • Weight gain
  • Fatigue
  • Dry skin
  • Dry, thinning hair
  • Being very sensitive to cold temperatures
  • Heavy or irregular menstrual periods or fertility problems
  • Slow heart rate
  • Depression

Symptoms of hyperthyroidism (too much thyroid hormone) may include:

  • Anxiety, nervousness, irritability
  • Weight loss, even though your appetite may increase
  • Shaky hands, muscle weakness
  • Sweating or being very sensitive to heat
  • Arrhythmia (a problem with the rate or rhythm of your heartbeat)
  • Frequent bowel movements (poops)
  • Goiter (an enlarged thyroid gland)
  • Trouble sleeping and fatigue

What happens during a thyroxine 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 thyroxine 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 health care 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).” To fully understand your results, your provider will often need to compare them with the results of other thyroid tests.

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

  • Hypothyroidism, which may be caused by Hashimoto disease or another medical condition that causes your thyroid to make too little T4
  • Thyroiditis (thyroid inflammation)
  • Side effects from certain drugs
  • Congenital hypothyroidism (a condition that affects babies from birth)

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

  • Hyperthyroidism, which may be caused by Graves disease or another medical condition that causes your thyroid to make too much T4
  • Thyroiditis (thyroid inflammation)
  • Toxic goiter (an enlarged thyroid with areas that make extra thyroid hormone)
  • Toxic thyroid nodule (a single lump on the thyroid that makes extra thyroid hormone)
  • Too much iodine in your diet
  • A noncancerous (not cancer) tumor in your pituitary gland