T4 (Thyroxine) Blood Test

What Is a T4 Blood Test and Why Is It Done?

A thyroxine (thy-ROK-​seen) (T4) blood test measures the amount of the T4 hormone in the blood. T4 is made by the thyroid gland and helps control metabolism and growth.

Doctors order a T4 test when they’re checking to see how well the thyroid gland is working. They may do the test if kids have symptoms that suggest there might be a thyroid disorder, like hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid).

Newborns are routinely screened for congenital (present at birth) hypothyroidism, which can cause serious medical problems if it’s not treated.

What Kinds of T4 Does the Test Check?

There are two types of T4 in the blood:

• bound T4, which attaches to proteins that keep the hormone in the bloodstream until it’s needed
• free T4, which isn’t attached to proteins and can enter parts of the body that need it

A total T4 blood test measures the entire amount of thyroxine in the blood (bound T4 and free T4). A free T4 blood test measures just free T4.

Top Things to Know

• The T4, or thyroxine, blood test checks the levels of a hormone in the blood called T4.
• The test can help show if there’s a thyroid problem, like hypothyroidism or hyperthyroidism.
• Kids might need to stop taking certain medicines or not eat or drink before the blood test.
• Your doctor can explain what it means if T4 levels are too high or too low.

How Can I Help My Child Prepare for a Blood Test?

To help kids get ready for a blood test, find out if they need to fast (not eat or drink) or should stop taking medicines before the test. Explain what to expect during the test. If your child is nervous about it, work together on ways to stay calm. Deep breathing or even imagining being in a place that makes your child feel safe and relaxed can help.

Tell the doctor about any medicines your child takes because some can affect the test results.

What Do T4 Total Test Results Mean?

Your doctor will explain what the results of the T4 test mean for your child. What’s “normal” varies based on things like age and gender. Taking certain medicines or being very sick can also change the test results.

High T4 Level

A high T4 level may be due to things like:

• hyperthyroidism
• Graves disease
• thyroid gland inflammation (thyroiditis)
• goiter or thyroid nodules (growths in the thyroid gland)
• too little or too much iodine in the diet

Low T4 Level

A low T4 level can happen with things like:

• hypothyroidism
• thyroiditis
• Hashimoto thyroiditis (hah-she-MOE-to thy-roy-DYE-tiss), where the immune system attacks the thyroid
• congenital hypothyroidism
• too little or too much iodine in the diet
• thyroid cancer treatment

What Else Should I Know?

When kids have a T4 blood test, they’ll likely have other thyroid blood tests too, such as:

• thyroid stimulating hormone (TSH) test
• T3 (triiodothyronine) total blood test

If you have questions about the T4 test or what the results mean, talk to your doctor.