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What Is a Blood Test?

By taking and testing a small sample of a person’s blood, doctors can check for many kinds of diseases and conditions. Blood tests help doctors check how the body’s organs are working and see if medical treatments are helpful.

To help your child 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 anxious about it, work together on ways to stay calm.

What Is T4?

T4, or thyroxine, is a hormone that is made by the thyroid gland and helps control metabolism and growth. 

T4 may be referred to as:

  • total T4, which is the entire amount of thyroxine in the blood, including bound T4, which attaches to proteins
  • free T4, which is only free T4, which isn’t attached to proteins

Why Are T4 Tests Done?

Doctors order T4 tests when they’re checking to see how well the thyroid is working. Doctors may order T4 tests if symptoms suggest a thyroid disorder such as 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.

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

What if I Have Questions?

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

Medically reviewed by: Larissa Hirsch, MD
Date reviewed: June 2023