What to Know about the Free Thyroxine (T4) Index Test

The free thyroxine index (FTI) test, also called the T4 index test, is a blood test used to diagnose thyroid disorders. Thyroxine (T4) is a thyroid hormone.

The test measures how much thyroxine is in your blood to help determine whether your thyroid gland is underactive (hypothyroidism) or overactive (hyperthyroidism). A normal free thyroxine index range is between .7 and 1.9 nanograms per deciliter.

This article discusses the free thyroxine index test, why it's done, and what the results can tell you.

Purpose of the Free Thyroxine Index 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.

Your healthcare provider may order an FTI and other thyroid tests if you have symptoms that could indicate thyroid disease. Other risk factors that could influence this decision include:

• Being female
• Being under the age of 40
• Having family members with thyroid disorders

The other thyroid tests include thyroid-stimulating hormone (TSH) and triiodothyronine (T3). The results of these tests are analyzed together to help with a diagnosis.

The FTI is not itself a separate blood test. It is calculated from the results of the total T4 test and the T3 resin uptake test. Because the FTI is often made inaccurate by medical conditions that change blood protein levels, it is not used very much in clinical medicine today. Instead, the free T4 level is now measured directly.

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 of Thyroid Blood Tests

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 healthcare provider may have given you.

Tell your healthcare provider 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:

• Some cancer drugs
• Thyroid medications (you may need to wait until after the test to take your daily medication)
• Some supplements, particularly biotin supplements, which can alter test results

Pregnancy can also affect your thyroid hormone levels, so make sure your healthcare provider 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 healthcare provider'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 healthcare provider 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 healthcare provider 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 healthcare provider is ordering numerous tests. A full set of thyroid tests may cost $500 or more.

Your healthcare provider'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 healthcare professionals who will perform a blood draw. They may ask you to confirm certain information, such as your name, birth date, healthcare provider 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.

Blood Draw

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 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 healthcare provider'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 Free T4 Results

In an adult, the typical range for free T4 is between .7 and 1.9 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.

Follow-Up

After the results come back, your healthcare provider 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 healthcare provider's office. If you're diagnosed with a thyroid disorder, then it's time to talk to your practitioner about treatment options.

Summary

The free T4 index (FTI) is a blood test that can help diagnose a thyroid condition such as Hashimoto's thyroiditis or Graves' disease. 

The free thyroixine index test is typically done alongside other thyroid blood tests including the TSH and T3 tests. The results of these tests are analyzed together to help your healthcare provider arrive at a diagnosis.