What is a triiodothyronine (T3) test?

This test measures the level of triiodothyronine (T3) in your blood. T3 is one of two major hormones made by your thyroid, a small, butterfly-shaped gland located near the throat. The other hormone is called thyroxine (T4.) T3 and T4 work together to regulate how your body uses energy. These hormones also play an important role in controlling your weight, body temperature, muscle strength, and nervous system.

The T3 hormone comes in two forms:

• Bound T3, which attaches to protein
• Free T3, which does not attach to anything

A test that measures both bound and free T3 is called a total T3 test. Another test called free T3 just measures free T3. Either test may be used to check T3 levels. If T3 levels are not normal, it can be a sign of thyroid disease.

Other names: thyroid function test; total triiodothyronine, free triiodothyronine, FT3

What is it used for?

A T3 test is most often used to diagnose hyperthyroidism, a condition in which the body makes too much thyroid hormone.
T3 tests are frequently ordered with T4 and TSH (thyroid-stimulating hormone) tests. A T3 test may also be used to monitor treatment for thyroid disease.

Why do I need a T3 test?

You may need a T3 test if you have symptoms of hyperthyroidism. These include:

• Anxiety
• Weight loss
• Tremors in the hands
• Increased heart rate
• Bulging of the eyes
• Trouble sleeping
• Fatigue
• Low tolerance for heat
• More frequent bowel movements

What happens during a T3 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?

You don't need any special preparations for a T3 blood test. Your health care provider will let you know if you need to stop taking any medicines before your test. Certain medicines can raise or lower T3 levels.

Are there any risks to the test?

There is very little risk to having a blood test. You may have slight pain or bruise at the spot where the needle was put in, but most symptoms go away quickly.

What is T4?

T4, also called thyroxine, is the main form of thyroid hormone made by the thyroid gland. Most T4 is bound to proteins, while a small proportion is unbound, or free. Free T4 testing measures T4 that circulates through the blood and is available to enter body tissues and act upon them. Total T4 testing measures the total amount of T4, including free T4 and T4 that is bound to proteins.

T4 is measured by taking a blood sample. A low T4 test result may indicate an underactive thyroid gland or problems with its stimulation by the pituitary gland. A high level of T4 may be a sign of an overactive thyroid, called hyperthyroidism. A T4 test may be ordered if thyroid problems are suspected or to follow up after an abnormal thyroid-stimulating hormone (TSH) test result.

T4 testing helps your doctor evaluate how well your thyroid is working. This test may be ordered to:

• Follow up on an abnormal TSH test result
• Diagnose hyperthyroidism, a condition involving an overactive thyroid gland
• Diagnose hypothyroidism, a condition in which the thyroid gland is underactive
• Monitor T4 levels in patients on thyroid replacement therapy
• Screen for an underactive thyroid gland in newborn babies
• Evaluate other disorders, such as goiters, thyroid nodules, and problems with the pituitary or hypothalamus gland

What does the test measure?

T4 or thyroxine is the major hormone made in your thyroid gland. The name T4 derives from the fact that the thyroxine molecule has four iodine atoms attached to it.

Thyroid hormones are critical to brain development during infancy and for many essential body processes during adulthood. Usually, your body has large stores of T4 circulating in your blood. However, most of your circulating T4 is bound to proteins and is not available for immediate use by the body.

Free T4 testing measures only the active form of thyroid hormone, which is unbound and can directly enter cells and affect them. Total T4 testing measures both bound and unbound T4 in the blood.

When should I get T4 testing?

There are a number of different reasons why a doctor might order a T4 test. Often it is ordered to follow up on an abnormal TSH test result.

It may also be used when a patient has symptoms that may be related to a thyroid disorder. Underactive thyroid (hypothyroidism) and overactive thyroid (hyperthyroidism) are both common diseases. If you are having symptoms of these disorders, your doctor may order a T4 test in combination with other thyroid function tests, such as TSH and T3. This series of tests is also called a thyroid panel.

Hypothyroidism affects nearly 5% of adolescents and adults in the United States. Symptoms of hypothyroidism may include:

• Fatigue
• Weight gain
• Feeling cold
• Painful joints and muscles
• Dry skin
• Thin and/or dry hair
• Slow heart rate
• Constipation
• Depression
• Irregular menstrual periods
• Fertility problems

Another common thyroid disorder is hyperthyroidism, which affects slightly more than 1% of people in the U.S.

• Weight loss
• Anxiety
• Insomnia
• Irritability
• Difficulty tolerating heat
• Mood swings
• Rapid or irregular heartbeat
• Loose, frequent bowel movements

There are additional reasons why a doctor might check a patient’s T4, such as monitoring the effectiveness of treatment with replacement thyroid hormones, screening newborns for congenital hypothyroidism, and evaluating possible problems with the hypothalamus or pituitary gland.