Patient education: T3 tests (The Basics)

What is a T3 test? — 

This is a type of blood test. It measures the amount of T3 in the blood.

The thyroid gland makes 2 different hormones: T3 and T4 (figure 1). After the thyroid releases T4, cells in some of the body's organs can turn it into T3. T3 is also called "triiodothyronine."

Some T3 in the body is "bound," meaning it attaches to proteins in the blood. The remaining T3 is "free." A blood test can measure "total" (bound and free) T3 or free T3 only.

If your T3 level is too high or too low, it might mean you have a thyroid problem.

Another test, called a "thyroid-stimulating hormone" ("TSH") test, is usually the only one needed to check how well the thyroid is working. TSH tells the thyroid to release T3 and T4. In some cases, measuring both TSH and free T4 can be useful. Total or free T3 is measured much less often.

Why might I get a T3 test? — 

Your doctor or nurse might want you to have a T3 test:

●If you had a TSH test, and the results were lower than normal

●If you have symptoms of hyperthyroidism – This is when the thyroid makes too much thyroid hormone. It can cause anxiety, trouble sleeping, shaking or sweating, and weight loss.

●To check how treatment for hyperthyroidism is working

How do I prepare for a T3 test? — 

You need to get a "blood draw." Your doctor or nurse will tell you where to go for this.

It might help to wear a short-sleeve shirt to your blood draw appointment. This makes it easier for the person drawing your blood to get to your arm.

What happens during a blood draw? — 

A needle is used to take a small amount of blood from your arm (figure 2). Collecting the blood only takes a few minutes. The blood is then tested in a lab.

Tell the person who takes your blood:

●If you take "blood thinner" medicines or if you have a bleeding problem – They will make sure your bleeding is under control before you leave.

●If you have a latex allergy – Some of the supplies used for blood draws might contain latex.

●If you have a preferred arm to use

Most of the time, getting blood taken does not cause problems. You might have a little soreness or bruising where the needle went in.

What do my results mean? — 

Your doctor or nurse will tell you when to expect your results, and will contact you with the results. Or if you use an online "patient portal," you might get an alert there when your results are ready.

If your T3 test shows any abnormal results, your doctor or nurse will talk to you about what to do next. They might need to do more tests to figure out the cause.

If you do have a health problem, your doctor will work with you to come up with a plan for treatment.

More on this topic

Patient education: Thyroid-stimulating hormone test (The Basics)
Patient education: Hyperthyroidism (overactive thyroid) (The Basics)
Patient education: Thyroglobulin test (The Basics)

Patient education: Hyperthyroidism (overactive thyroid) (Beyond the Basics)