What Is a Total Thyroxine (T4) Test?

The total thyroxine test is used to diagnose thyroid disorders. Thyroxine (T4) is a thyroid hormone, and the test measures how much is in your blood. Some thyroid diseases are tied to too little T4, and others are tied to too much.

Other names for this test are a total T4 test, total T4 concentration, and a thyroxine screen.

Purpose of Test

The thyroid is a gland in the front of your throat that makes hormones and regulates a variety of important functions including energy use, weight, body temperature, and mood.

T4 functions in your body in a couple of forms. One form bonds with a protein to perform certain tasks and another form doesn't, which allows it to perform different jobs. The total T4 test checks total T4 levels, which includes both forms and free T4, which is the form not bonded with a protein.

The level of total T4 is greatly dependent on the amount of binding protein in the blood, and the amount of protein is influenced by many nonthyroid conditions. So the total T4 measurement may not accurately reflect the condition of the thyroid. For this reason, the free T4 test, which measures only the active, unbound portion of T4 in the blood, is considered a much superior test, and it has largely supplanted the total T4 test in clinical medicine.

Your healthcare provider may order a total T4 test if you have symptoms that could indicate hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid), especially if you have other risk factors such as being female and over the age of 60 or you have family members with thyroid disorders.

These generally include tests of thyroid stimulating hormone (TSH) and triiodothyronine (T3). Tests to look at antibodies that could indicate autoimmune disease may also be done at the same time or may be done as a follow-up to abnormal thyroid-function tests.

Low levels of 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 T4 and other thyroid tests are performed on blood samples. Taking blood samples is fairly quick, simple, and low risk.

Risks

The most common risks people face from a blood test is slight pain or mild 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, let someone at the medical center know. Most facilities are well equipped to deal with this reaction, which usually involves lying down for a while and drinking 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 is drawing your blood know before the test if you've had bad reactions to blood draws in the past, you have a bleeding disorder or are taking blood-thinning medications, or if your skin tears or bruises easily. They should be able to mitigate any risk these elements may create.

Before the Test

Blood for T4 and other thyroid tests can generally be drawn at any time of day. It 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.

• You may need to take a break from certain medications, if your endocrinologist advises it. These include:
• Steroids
• Some cancer drugs
• Thyroid medications (unless you're being checked for how effective the medication is)

Other drugs may impact results as well, and so could supplements, so make sure your healthcare provider is aware of everything you're taking. Biotin, in particular, interferes with thyroid assays. Patients should stop biotin three days before a thyroid test, especially if the dosage is exceeding 5,000 mcg per day.

Birth control medications as well as 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. If the blood is being drawn in your healthcare provider's office during an appointment, it should add very little time to the appointment length.

Going to a lab for the test can require a larger time commitment. If you have a scheduled time for your test, be sure to arrive early enough to check in and pay any copays that may be due at the time. If you're anxious about the test, you may also want to give yourself a few minutes to sit calmly 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. Your arm will need to be exposed, so sleeves that are short or can be pushed up easily are best.

In case of a dizzy spell, you may want to wear flat shoes and clothing that's not tight and constricting.

Food and Drink

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

If your healthcare provider is ordering other tests at the same time, however, some could require fasting. Be sure to follow all of the instructions you're given by the practitioner and/or the lab performing the test.

Cost and Health Insurance

Typically, thyroid function tests are covered by insurance when they're deemed medically necessary. 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 a total T4 test can range from $45 to about $130. If you're not insured, you may face additional charges on top of that, especially if your healthcare provider is ordering more than one test. A full set of thyroid tests may cost more than $500.

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

During the Test

Before the blood draw, the nurse or phlebotomist 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 help prevent errors.

Pre-Test

You'll be asked to expose your arm, and the insertion area 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.

Throughout the Test

Once a good vein is identified, the needle will be inserted. The band will be released to get the blood flowing again, and the vial attached to the needle will fill up. Depending on how many tests are ordered, the nurse/phlebotomist may need to change vials one or more times.

After the requisite blood has been drawn, the insertion site will be bandaged.

Post-Test

Most of the time, you can leave right away after a blood draw. If you have any kind of negative reaction, let someone at the facility know so you can get the proper care. Usually, even with a bad reaction, people are OK 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, be sure to contact your healthcare provider.

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 adults, the typical range for free T4 is between .9 and 2.o nanograms per deciliter. Total T4 tends to vary more with age.

An abnormally high T4 level may indicate hyperthyroidism, thyroiditis, or goiter. It may lead to additional tests for Graves' disease.

An abnormally low T4 level can be a sign of 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

Once you have the results, your healthcare provider may want you to come back in to discuss further testing or treatment options, depending on whether a diagnosis has been made. 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 are diagnosed with a thyroid disorder, then it's time to look into treatment options as early treatment is key.

A Word From Verywell

Thyroid disorders are common (especially in women) and highly treatable. Many thyroid diseases are managed by taking medication and possibly making some dietary changes. While being diagnosed with a thyroid disorder can be scary, the upside is that it means you can start treating and managing the condition, which should mean feeling and functioning better.