T3 Total (Triiodothyronine) Blood Test

T3 (Triiodothyronine) Total Blood Test

What Is a T3 Total Blood Test and Why Is It Done?

A triiodothyronine (try-eye-oh-doh-THIGH-ruh-neen) (T3) total blood test measures the amount of the T3 hormone in the blood.

Doctors order T3 total tests when they’re checking how a person’s thyroid gland is working. They may do the test when a child's symptoms or previous blood tests suggest a thyroid problem. The T3 test is useful in diagnosing hyperthyroidism. In this condition, the thyroid makes too much of certain hormones, causing symptoms like fast heart rate, weight loss, trembling, and sweating.

What Is T3?

The hormone T3 helps control many body functions, including growth, body temperature, and heart rate. The thyroid gland makes some T3. But most is made in parts of the body where T4 (the major hormone made by the thyroid) chemically changes to T3.

What Kinds of T3 Does the Test Check?

The T3 total test, which is the most common type of T3 blood test, measures the two types of T3 in blood:

• bound T3, which attaches to proteins that keep the hormone in the bloodstream until it’s needed
• free T3, which isn’t attached to proteins and can enter parts of the body that need it

Top Things to Know

• The triiodothyronine (T3) total blood test checks the levels of a hormone in the blood called T3.
• The test can help show if there’s a thyroid problem, like 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 T3 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 T3 Total Test Results Mean?

Your doctor will explain what the results of the T3 total test mean for your child. “Normal” T3 levels vary based on things like age and gender.

High T3 Level

A high T3 level is often due to hyperthyroidism, or an overactive thyroid gland, which can be caused by things like:

• Graves disease
• thyroid gland inflammation (thyroiditis)
• thyroid nodules (growths in the thyroid gland)

Low T3 Level

A low T3 level can happen with hypothyroidism (an underactive thyroid gland), but doctors will need to do more tests to be sure. Other things can lower T3 as well, like taking certain medicines or being very sick.

What Else Should I Know?

When kids have a T3 total blood test, they’ll likely have other thyroid blood tests too, such as a thyroid stimulating hormone (TSH) test and one that measures the hormone T4.

If you have questions about the T3 test or what the results of the test mean, talk to your doctor.