What Is a Free Thyroxine Test?

A free thyroxine test, often called a free T4 test, measures the amount of free thyroxine in your blood. Thyroxine (also called T4) is one of the major hormones produced by your thyroid. When T4 is created, some of the hormone gets bound to proteins while the rest circulates freely.

The free T4 that is being tested for is unbound and is available to the cells in the body to be used for things like heart and digestive function, metabolism, brain development, and bone and muscle health.

Purpose of Test

A doctor may order a free T4 test if a patient is having symptoms of a thyroid disease such as weight loss, a rapid heart rate, and sweating associated with hyperthyroidism (when your thyroid produces too many hormones). A free T4 test may also be performed if you are experiencing weight gain, feeling cold, or have a sense of general fatigue that might be associated with hypothyroidism (when the thyroid isn’t producing enough hormones).

Even if you aren’t showing any signs or symptoms of thyroid disease, your doctor may want to run a free T4 test if you have a family history of thyroid conditions. A free T4 test is considered the most accurate test for assessing T4, as opposed to a total T4 test.

There are similar tests that are used to evaluate thyroid function, including a thyroid stimulating hormone (TSH) test and a triiodothyronine (T3) test. Your TSH is the hormone produced by the pituitary gland that inform the thyroid how much T4 and T3 hormones to produce in the body.

A T3 test may also be done alongside a free T4 test to help inform a diagnosis, as the measurement of free T4 together with T3 may be useful in identifying hyperthyroidism.

Risks and Contraindications

Since a T4 test is done simply with a blood draw, there are few to no risks of the procedure, aside from the risks you may have when getting blood drawn. You may have a headache or feel slightly queasy during the draw and notice tenderness, redness, or slight bruising at the site where the draw took place. If you feel nervous about having blood taken, make sure to tell the technician before they start the test so they can be aware and help make you feel more comfortable during and after the test.

For example, a small snack and drink after the blood draw may help with lightheadedness and any anxiety you may have that is associated with needles.

Before the Test

There are no preparations that need to be done ahead of time with a free T4 test, but you do need to tell your doctor any medications, vitamins, or herbs you are taking ahead of time, as these can interfere with the results of the test. Most doctors will recommend having your blood drawn before you take any medications if possible.

If you’re suffering from an illness such as a cold, your doctor may also choose to postpone the blood draw until you’ve recovered. The test itself should only take a few minutes and the results should be available a few days after your doctor sends your blood vial off to the lab to be examined.

Location

A free T4 test will likely take place right in your doctor’s office. In certain cases, you may have to go to a separate lab to get your blood drawn at a time that is convenient for you.

What to Wear

Remember to wear a shirt that can be easily rolled up, or a sweatshirt with a t-shirt underneath so that you can remove it for the blood draw. It’s important for the technician performing the blood draw to be able to access your arm to find the best vein from which to take blood with minimal discomfort to you.

Food and Drink

Unless specified by your doctor, hold off on any vitamins or medications until after your free T4 test. Supplements containing biotin should be ceased four days before the test. Make sure you eat a healthy meal or snack an hour or two before your free T4 test to prevent getting dizzy or lightheaded during the blood draw.

Cost and Health Insurance

When medically necessary, a free T4 test is usually covered by health insurance. Depending on your plan coverage you may have to pay a coinsurance fee, which is usually anywhere from 10% to 50%.

Interpreting Results

Once your doctor sends the blood draw off to the lab for analysis, the results should take no more than a few days to a week to come in. The results of a free T4 test vary depending on your age, gender, and health history. A normal free T4 range in adults is .8 to 2.8 nanograms per deciliter (ng/dL). Higher T4 levels could indicate you have an overactive thyroid as lower T4 levels could indicate you have an underactive thyroid.

The results can also help show if the current prescribed treatment course for a previously diagnosed thyroid disease is working successfully. Free T4 tests are typically evaluated with other diagnostic thyroid tests, including a TSH test and sometimes a T3 test. For example, if the results show a high TSH level and a low free T4 level, this indicates hypothyroidism due to a defect in the thyroid.

A low TSH level and a low free T4 level is likely hypothyroidism due to a defect in the pituitary gland, and a low TSH level with a high free T4 level can be a hyperthyroidism due to a defect in the thyroid.

While there are no lifestyle changes that can increase or decrease free T4 in the body, medication can be prescribed to help adjust the amount entering the bloodstream to help ensure your body is operating with optimal levels of the hormone. In certain cases such as pregnancy, free T4 levels can increase as a result, which is not necessarily a sign of thyroid disease. In cases like these, free T4 levels return to normal after the birth.

A Word From Verywell

Reading free T4 test results alone without a comprehensive thyroid panel, including a TSH test, can give you an incomplete look into your thyroid function. This is because even if you see abnormal free T4 levels it’s important to understand why those levels are abnormal, whether it’s an issue with your thyroid or with your pituitary gland.

Results of a free T4 test are just a glimpse of how your thyroid is functioning overall, and it’s important to discuss all of the test results with your doctor so they can determine what a normal T4 level and range for your health history is, so together you can treat your specific thyroid disorder effectively with minimal side effects.