Thyroxine (T4) Test

What is a thyroxine (T4) test?

A thyroxine test helps diagnose disorders of the thyroid. The thyroid is a small, butterfly-shaped gland located near the throat. Your thyroid makes hormones that regulate the way your body uses energy. It also plays an important role in regulating your weight, body temperature, muscle strength, and even your mood. Thyroxine, also known as T4, is a type of thyroid hormone. This test measures the level of T4 in your blood. Too much or too little T4 can indicate thyroid disease.

The T4 hormone comes in two forms:

Free T4, which enters the body tissues where it’s needed
Bound T4, which attaches to proteins, preventing it from entering body tissues

A test that measures both free and bound T4 is called a total T4 test. Other tests measure just free T4. A free T4 test is considered more accurate than a total T4 test for checking thyroid function.

Other names: free thyroxine, free T4, total T4 concentration, thyroxine screen, free T4 concentration

What is it used for?

A T4 test is used to evaluate thyroid function and diagnose thyroid disease.

Why do I need a thyroxine test?

Thyroid disease is much more common in women and most often occurs under the age of 40. It also tend to run in families. You may need a thyroxine test if a family member has ever had thyroid disease or if you have symptoms of having too much thyroid hormone in your blood, a condition called hyperthyroidism, or symptoms of having too little thyroid hormone, a condition called hypothyroidism.

Symptoms of hyperthyroidism, also known as overactive thyroid, include:

Anxiety
Weight loss
Tremors in the hands
Increased heart rate
Puffiness
Bulging of the eyes
Trouble sleeping

Symptoms of hypothyroidism, also known as underactive thyroid, include:

Weight gain
Fatigue
Hair loss
Low tolerance for cold temperatures
Irregular menstrual periods
Constipation

What happens during a thyroxine 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 thyroxine blood test. If your health care provider has ordered more tests on your blood sample, you may need to fast (not eat or drink) for several hours before the test. Your health care provider will let you know if there are any special instructions to follow.

Are there any risks to the test?

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

What do the results mean?

Your results may come in the form of total T4, free T4, or a free T4 index.

The free T4 index includes a formula that compares free and bound T4.
High levels of any of these tests (total T4, free T4, or free T4 index) may indicate an overactive thyroid, also known as hyperthyroidism.
Low levels of any of these tests (total T4, free T4, or free T4 index) may indicate an underactive thyroid, also known as hypothyroidism.

If your T4 test results are not normal, your health care provider will likely order more thyroid tests to help make a diagnosis. These may include:

T3 thyroid hormone tests. T3 is another hormone made by the thyroid.
A TSH (thyroid-stimulating hormone) test. TSH is a hormone made by the pituitary gland. It stimulates the thyroid to produce T4 and T3 hormones.
Tests to diagnose Graves’ disease, an autoimmune disease that causes hyperthyroidism
Tests to diagnose Hashimoto’s thyroiditis, an autoimmune disease that causes hypothyroidism

Is there anything else I need to know about a thyroxine test?

Thyroid changes can happen during pregnancy. Although it is not common, some women can develop thyroid disease during pregnancy. Hyperthyroidism happens in about 0.1% to 0.4% of pregnancies, while hypothyroidism happens in approximately 2.5% of pregnancies.

Hyperthyroidism, and less often, hypothyroidism, may remain after pregnancy. If you develop a thyroid condition during pregnancy, your health care provider will monitor your condition after your baby is born. Also, if you have a history of thyroid disease, be sure to talk with your health care provider if you are pregnant or are thinking of becoming pregnant.

References

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Hinkle J, Cheever K. Brunner & Suddarth’s Handbook of Laboratory and Diagnostic Tests. 2^nd Ed, Kindle. Philadelphia: Wolters Kluwer Health, Lippincott Williams & Wilkins; c2014. Thryoxine, Serum 485 p.
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National Institute of Diabetes and Digestive and Kidney Diseases [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; Thyroid Tests; 2014 May [cited 2017 May 22]; [about 3 screens]. Available from: https://www.niddk.nih.gov/health-information/diagnostic-tests/thyroid
National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; What Are the Risks of Blood Tests? [updated 2012 Jan 6; cited 2017 May 22]; [about 5 screens]. Available from: https://www.nhlbi.nih.gov/health-topics/blood-tests#Risk-Factors
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