Patient education: Free T4 test (The Basics)

What is a free T4 test? — This is a type of blood test. It measures the amount of free T4 in the blood.

T4 is 1 of the hormones the thyroid gland makes (figure 1). It is also called "thyroxine." Some T4 in the body is "bound," meaning it attaches to proteins in the blood. The remaining T4 is "free."

If your free T4 level is too high or too low, it might mean that you have a problem with your thyroid.

Another test, called a TSH test, is usually the only test needed to check how well your thyroid is working. TSH stands for "thyroid-stimulating hormone." It tells the thyroid to release the thyroid hormones T4 and T3. In some cases, measuring both TSH and free T4, or even just free T4, can be useful.

Why might I get a free T4 test? — Your doctor or nurse might want you to have a free T4 test if:

●You had a TSH test, and the results were abnormal.

●You are still having symptoms of a thyroid problem even if your TSH test was normal. Examples include:

•Hypothyroidism – This is when the thyroid makes too little thyroid hormone. Symptoms can include feeling tired or weak, getting cold easily, thinning hair, weight gain, and constipation.

•Hyperthyroidism – This is when the thyroid makes too much thyroid hormone. Symptoms can include anxiety, trouble sleeping, shaking or sweating, and weight loss.

●You have risk factors for thyroid disease – Testing people who don't have symptoms is called "screening." Free T4 tests are only used for screening in certain situations. Most of the time, a TSH test is done instead.

How do I prepare for a free T4 test? — For this 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? — For the 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 that 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 free T4 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 what the cause is.

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: Hypothyroidism (underactive thyroid) (The Basics)
Patient education: Hyperthyroidism (overactive thyroid) (The Basics)
Patient education: Thyroiditis (The Basics)

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