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Blood Test: T3 Total (Triiodothyronine)

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 T3?

T3 (triiodothyronine) is a hormone that helps control many body functions, including growth, body temperature, and heart rate. Some T3 is made by the thyroid gland. But most is made in parts of the body where T4 (the major thyroid hormone made by the thyroid) chemically converts to T3.

Two types of T3 are in blood:

  • bound T3, which attaches to proteins that help carry the hormone through the body
  • free T3, which isn’t attached to proteins

The T3 total test, which is the most common type of T3 blood test, measures levels of both the bound and free types of T3.

Why Are T3 Total Tests Done?

Doctors order T3 total tests when they’re checking a person’s thyroid function. Doctors may order the T3 test when a child's symptoms or previous blood tests suggest a thyroid problem. The T3 test is particularly useful in diagnosing hyperthyroidism. In this condition, the thyroid overproduces hormones, causing symptoms such as a fast heart rate, weight loss, trembling, and sweating.

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 T3 test or what the results of the test mean, talk to your doctor.

Reviewed by: Larissa Hirsch, MD
Date Reviewed: Jun 1, 2023

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