What Is a Total Triiodothyronine (T3) Test?

What to Expect With This Test

The total triiodothyronine (T3) test is one of several tests used to diagnose thyroid disorders. T3 is an active thyroid hormone that your body creates by converting thyroxine (T4) into triiodothyronine. The 3 and 4 in the names represent the number of iodine atoms it has.

The T3 test simply measures how much of this hormone is present in your blood. If the level is either abnormally high or abnormally low, it can be an indicator of thyroid disease. However, this test is generally more useful for diagnosing hyperthyroidism (overactive thyroid) than hypothyroidism (underactive thyroid), and it's typically combined with measures of other thyroid hormone levels rather than taken alone.

Alternate names for this test include T3 radioimmunoassay, toxic nodular goiter—T3, thyroiditis—T3, thyrotoxicosis—T3, and Graves' disease—T3.

Purpose of Test

The thyroid is a butterfly-shaped gland in the front of your throat. It makes hormones and regulates numerous things, including your body's energy use, body temperature, weight, and mood.

T3 exists in your body in two different forms:

  • free T3, which is the active form and is bound to a protein
  • reverse T3, the inactive form, which attaches to thyroid receptors but can't activate them

The total T3 test looks at both of these forms. Your doctor may choose to perform separate tests for free T3 and reverse T3.

Total T3 tests are generally ordered when you have symptoms that are consistent with thyroid disease, especially if you have an elevated risk. Risk factors include being female, being under the age of 40, and having family members who are diagnosed with thyroid disorders.

Your doctor will likely order other measurements of thyroid function at the same time as T3 test including tests of thyroid stimulating hormone (TSH) and T4. Tests to look for antibodies that indicate autoimmune thyroid disease, including Hashimoto's thyroiditis and Graves' disease.

The T3 test, as well as other thyroid tests, are performed on blood samples. The process for drawing blood for these tests is generally quick, simple, and low-risk.

Risks and Contraindications

You may experience a little bit of pain or mild bruising after the blood draw, but this shouldn't last long. Sometimes, people may experience dizziness, nausea, or ringing in the ears during or immediately following a blood draw. Let your healthcare professional know if you feel any of these symptoms. Usually, this can easily be handled by laying down for a few minutes and hydrating with water.

If you've had negative reactions to past blood draws, it may be safest to arrange for someone to drive you to and from the test facility.

Let the person who's drawing your blood know before the test if you have a history of bad reactions to blood draws, you have a bleeding disorder or are taking medications that thin your blood, and/or your skin bruises or tears easily. They should be able to mitigate any risk these factors create.

Before the Test

Blood for T3 and other thyroid tests can generally be drawn at any time of day. Unless other tests that do require fasting are ordered at the same time, it shouldn't matter when you've eaten last.

When you arrive for the test, be sure to have your insurance card and any written orders your doctor may have given you.

Some medications can interfere with thyroid hormone levels and test results. Talk to your doctor about whether you need to take a break from any medications prior to the test.

Known problem medications include:

  • Hormone-containing drugs (i.e., birth control pills, estrogen, androgen)
  • Steroids
  • Some drugs for treating cancer
  • Thyroid medications

Other drugs and supplements may impact your results as well, so be sure your doctor knows about everything you're taking.

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

Timing and Location

The blood draw itself shouldn't take more than a few minutes. If you're getting a lot of tests done, it could take slightly longer. Some doctor's offices may perform the blood draw in conjunction with an appointment. In that case, it should add very little time to the appointment length.

Going to a lab for the test can take more time. If you have a scheduled appointment for your test, be sure to arrive early enough to check in and take care of any co-pays that may be due. If you're nervous about the test, you may also want to give yourself a few minutes to sit and relax.

Labs are often busy and wait times may vary. If you're dropping in, it can be a good idea to call ahead of time. Certain times of day, especially first thing in the morning, may be better than others.

What to Wear

You should be able to stay in your own clothing during the test. Your arm will need to be exposed, so short sleeves or those that can easily be pushed up above your elbow will work best.

In case you have a dizzy spell, flat shoes and clothing that is not tight or constrictive may be good choices.

Food and Drink

Any time you're having blood drawn, you should be sure you're well hydrated. It will make your veins easier to find and help avoid problems with needle insertion.

Since you don't need to fast before a T3 test, it's a good idea to have something in your stomach. It may help you avoid becoming nauseous.

Remember that other tests ordered at the same time may require fasting or other special instructions. Be sure to follow all of the instructions you're given to ensure accurate test results.

Cost and Health Insurance

Most often, thyroid function tests that are deemed medically necessary will be covered by insurance. However, it's never a bad idea to check with your insurance company ahead of time so you know what expenses you could face.

Laboratory costs for a total T3 test can range from $35 to around $100. If you're not insured, you may face additional charges on top of that, especially if your doctor is ordering more than one test. A full set of thyroid tests may cost $500 or more.

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

During the Test

A blood draw is most often performed by a nurse (in a doctor's office) or a phlebotomist (in a lab.) Before the test, they may ask you to confirm certain information such as your name, birth date, the doctor who ordered the test, and what you're being tested for. This is to make sure everything is labeled correctly.

Pre-Test

Once you've exposed your arm, the insertion area will be cleaned with alcohol. The nurse or phlebotomist will tie a band around your upper arm to trap your blood and get your veins to stand out. You may be asked to pump your fist to help with this process.

Throughout the Test

Once they identify a good vein, they'll insert the needle, which will probably cause a little bit of pain upon insertion (but this shouldn't last long). They'll then release the band to get the blood flowing again, and the vial attached to the needle should fill up. Depending on how many tests are ordered, they may need to change vials one or more times.

After enough blood has been drawn, they'll withdraw the needle and put a bandage over the insertion site.

Post-Test

Most of the time, you can leave right away after your blood is drawn.

If you have any kind of negative reaction, let someone at the facility know so they can take care of you properly. Usually, even with a bad reaction, it only takes a few minutes of rest to be okay to leave.

After the Test

You may notice a little tenderness and possibly bruising around the site where the needle was inserted, which typically goes away quickly. If you have any problems or questions, contact your doctor.

Managing Side Effects

If the site is noticeably sore, you can ice it or take over-the-counter pain medication. You shouldn't have any other lingering effects.

Interpreting Results

The normal ranges for adults are generally considered to be:

  • Total T3: between 60 and 180 nanograms per deciliter (ng/dL), or between .92 and 2.76 nanomoles per liter (nmol/L)
  • Free T3: between 130 and 450 picograms per deciliter (pg/dL), or between 2 to 7 picomoles per liter (pmol/L)

It's important to note that not every lab uses the same ranges or measurements.

An abnormally high level of T3 isn't generally a sign of thyroid disease by itself, but it is considered along with TSH and T4 levels.

A high T3 level may indicate:

  • Hyperthyroidism if accompanied by a low TSH level
  • Graves' disease if accompanied by a low TSH level and positive tests to detect certain antibodies
  • Toxic nodular goiter
  • Liver disease
  • A rare condition called T3 thyrotoxicosis

High T3 can also be due to pregnancy or the use of medications containing estrogen.

An abnormally low T3 level may indicate:

  • Hypothyroidism, if accompanied by a high TSH level
  • Hashimoto's thyroiditis, if accompanied by a high TSH level and positive tests to detect certain antibodies
  • Malnutrition/starvation
  • A severe short-term illness or some long-term illnesses

Follow-Up

Once your results are in, your doctor may want you to make an appointment to discuss further testing or treatment options, depending on whether a diagnosis has been made.

If you don't hear back about your results for a long time, or if you don't understand your results, contact your doctor's office. If you are diagnosed with a thyroid disorder, then it's time to start discussing your treatment options with your doctor.

A Word From Verywell

Any chronic diagnosis can be scary, but keep in mind that thyroid disorders are common and highly treatable. Often, they can be managed by taking medication and possibly making a few dietary changes. The good thing about a diagnosis is that it can lead to feeling better and being healthier with early treatment. Be sure to discuss your concerns with your doctor or healthcare professional so you can be assured you're on the right path for managing your health in the best way possible.

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