What Is a Free T4 Index Test?

What to Expect When Undergoing This Test

The Free T4 Index (FTI) is a blood test used to diagnose thyroid disorders. T4, also called thyroxine, is a thyroid hormone. The test measures how much of it is in your blood to help determine whether your thyroid gland is underactive (hypothyroidism) or overactive (hyperthyroidism).

Other names for this test include:

  • Thyroxine test
  • Free thyroxine test
  • FT4

If your doctor orders an FTI, it will probably be performed along with other thyroid tests, including thyroid stimulating hormone (TSH) and triiodothyronine (T3). The results are then analyzed together to help with a diagnosis.

Purpose of Test

The thyroid is located in the front of your throat and is shaped like a butterfly. Its purpose is to make hormones and regulate several important bodily functions, such as energy use, weight, body temperature, and mood.

In your body, T4 functions in two different forms. One form bonds with a protein to perform certain tasks and another form doesn't, which allows it to do different jobs.

The FTI test checks only for the "free" form, which is the form not bonded with a protein. It's considered the most useful level for evaluating thyroid function.

Instead of an FTI, your doctor may order a total T4 test, which looks at both forms. You'll be sent for an FTI and other thyroid tests if you have symptoms that could indicate a thyroid disease, especially if you have other risk factors, which include being female, being under the age of 40, and having family members with thyroid disorders.

Low levels of free T4 could lead to more testing to determine whether you have an autoimmune thyroid disease called Hashimoto's thyroiditis. High levels may lead to testing for Graves' disease, thyroiditis, or goiter.

The FTI and other thyroid tests are performed on blood samples, which are fairly quick and simple to obtain and very low risk.

Risks

The only risk most people face from a blood test is mild pain or a little bruising afterward, which should go away quickly.

Some people, especially those who are afraid of needles, may experience dizziness, nausea, or ringing in the ears during or immediately following a blood draw. If this happens to you, tell the person drawing your blood. Typically, they'll have you lie down for a while and drink some water.

If you have a history of negative reactions, you may want to arrange for someone to drive you to and from the test facility.

Let the nurse or phlebotomist who's drawing your blood know before the test if:

  • you've had prior bad reactions to blood draws
  • you have a bleeding disorder
  • you're taking blood-thinning medications
  • your skin tears or bruises easily

They can take steps to mitigate any risk these factors create.

Before the Test

Blood for FTI and other thyroid tests can generally be drawn at any time of day and doesn't require fasting (avoiding food) beforehand.

Be sure to have your insurance card and any written orders your doctor may have given you.

Tell your doctor about any medications you're taking and whether they could alter your test results. You may need to take a break from certain medications, including:

  • Hormone-containing drugs (such as birth control pills, estrogen, androgen)
  • Steroids
  • Some cancer drugs
  • Thyroid medications (you may need to wait until after the test to take your daily medication)

Other drugs may impact results, too, and some supplements can as well. Let your doctor know about everything you're taking.

Pregnancy can also affect your thyroid-hormone levels, so make sure your doctor knows if you are or could be pregnant.

Timing and Location

The blood draw itself should just take a few minutes. If you're getting multiple tests, it could take slightly longer.

Some doctor's offices will do the blood draw right there, during your appointment. In that case, it shouldn't add much time to the length of your appointment.

If you need to go to a lab for the test, it can require more time. If you have a scheduled appointment, arrive early enough to check in and pay any co-pays that may be due. If you're anxious about the test, you may also want to give yourself a few minutes to sit and relax.

Labs are often busy, so if you're dropping in, you may want to call ahead of time or aim for a time of day, such as early in the morning, when they tend to be less crowded.

What to Wear

You should be able to stay in your own clothing for the blood draw. Wear either short sleeves or long sleeves that can easily be pushed up past your elbow. Flat shoes and clothing that is not tight or constrictive are good choices in case you have a temporary dizzy spell.

Food and Drink

Any time you're having blood drawn, you want to be well hydrated. That helps your veins keep their shape, which means it's easier to find them and successfully insert the needle.

Since you don't need to fast before a T4 test, it's a good idea to have something in your stomach. It may help you avoid becoming nauseous.

However, if your doctor is ordering other tests at the same time, some could require fasting. Be sure to follow all of the instructions you're given by the doctor and/or the lab performing the test. If you do need to fast, try to take something with you so you can eat right after the test.

Cost and Health Insurance

Typically, thyroid function tests that are deemed medically necessary are covered by insurance. If you have any questions, be sure to call your insurance company ahead of time to see what, if any, out-of-pocket expenses you could face.

Laboratory costs for an FTI test can range from $45 to about $130. If you're not insured, you may face additional charges, especially if your doctor is ordering numerous tests. A full set of thyroid tests may cost $500 or more.

Your doctor's office, the insurance company, and the lab should be able to help you determine the cost before you get the tests.

During the Test

A nurse or phlebotomist are typically the ones who will perform a blood draw. They may ask you to confirm certain information, such as your name, birth date, doctor ordering the test, and what test you've been sent for. This is to ensure the right tests are performed on the right people.

Pre-Test

When it's time for the test, you'll be asked to expose your arm, and then the insertion spot will be cleaned with alcohol. The person performing the draw will tie a band around your upper arm to trap blood in your veins, which makes it easier to find a good vein and insert the needle. If your veins aren't standing out well, you may be asked to pump your fist.

During the Test

Once a good vein is found, the needle will be inserted. This is generally the most painful part of the test, but remember, it's only temporary. The band will then be released to get the blood flowing, and the vial attached to the needle will fill up. Depending on how many tests are ordered, you may need to fill up two or more vials. After enough blood has been drawn, the insertion site will be bandaged.

Post-Test

Most of the time, you can leave right away once the test is over. If you have any kind of negative reaction, speak up so you can get the proper care. Usually, even with a bad reaction, people are okay after a few minutes.

After the Test

You may have a little soreness and bruising around the site where the needle was inserted, which typically goes away in a short amount of time. If you have any problems or questions, contact your doctor's office.

Managing Side Effects

If the site is sore, you may want to ice it or take over-the-counter pain medication. You shouldn't have any other lingering effects from a simple blood draw.

Interpreting Results

In an adult, the typical range for free T4 is between .9 and 2.4 nanograms per deciliter. An abnormally high free T4 level may indicate hyperthyroidism, thyroiditis, or goiter. It may also lead to additional tests for Graves' disease.

An abnormally low free T4 level can point to hypothyroidism, a problem with your pituitary gland, malnutrition or iodine deficiency, or other illness. It may lead to further tests for Hashimoto's thyroiditis.

However, an abnormal T4 level alone doesn't usually lead to a diagnosis. Your doctor will likely analyze it along with T3 and TSH levels.

Follow-Up

After the results come back, your doctor may want you to come in to discuss further testing or treatment options, depending on your diagnosis.

If you don't hear back about your results in a timely manner, or if you don't understand what they mean, contact your doctor's office. If you're diagnosed with a thyroid disorder, then it's time to talk to your doctor about treatment options.

A Word From Verywell

Thyroid disorders are common and highly treatable. Many thyroid diseases are managed by taking medication and possibly making some dietary changes. While being diagnosed with any chronic condition can be scary, the bright side is that a diagnosis means you can start treating and managing the symptoms that sent you to the doctor in the first place. That's the first step toward feeling better.

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Article Sources
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