Free and Bound T-4
Does this test have other names?
Total T-4 concentration, thyroxine screen, free T-4 concentration
What is this test?
This is a blood test to measure your level of the hormone thyroxine, or T-4.
T-4 is made in your thyroid, a butterfly-shaped gland located near the base of your throat above your collarbones. In addition to T-4, your thyroid also makes T-3. Both hormones affect your energy level, mood, weight, and other important elements of your health.
T-4 is found in the body in two forms: free T-4 and bound T-4. Free T-4 travels into body tissues that use T-4. Bound T-4 attaches to proteins that prevent it from entering these tissues. More than 99% of T-4 is bound.
This test can show your doctor whether your thyroid gland is overactive, a condition called hyperthyroidism, or underactive, a condition called hypothyroidism.
Why do I need this test?
You may need this test if you have symptoms of thyroid problems.
Symptoms of hyperthyroidism, or too much thyroid activity, include:
Weakness in the arms and legs
Low tolerance for heat
Unexplained weight loss
More frequent bowel movements than usual
Eye irritation or bulging eyes; these are symptoms of Graves' disease, a common cause of hyperthyroidism
Enlarged breasts and erectile dysfunction in men
Symptoms of hypothyroidism, or less than normal thyroid activity, include:
What other tests might I have along with this test?
Your doctor may also order other tests to measure thyroid-related substances, including:
Thyroid stimulating hormone (TSH)
Thyroglobulin, which is used to make and store thyroid hormones
TSH receptor-stimulator antibodies, which is used to diagnose Graves' disease
Thyroid antiperoxidase antibodies and thyroglobulin antibodies, which are used to diagnose Hashimoto's thyroiditis
What do my test results mean?
Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.
Results are given in micrograms per deciliter (mcg/dL). The normal range for total T-4—both free and bound—varies by laboratory, but the usual range is 4.6 to 11.2 mcg/dL.
Free T-4 is usually measured two ways:
Free T-4, with normal ranges determined by the testing method the lab uses
Free T-4 index, a formula that includes total T-4 and a measurement called thyroid hormone-binding index. The normal range for the free T-4 index is 1.1 to 4.3 mcg/dL.
If your results show high total T-4 or a high free T-4 index, it means you may have hyperthyroidism. If your results show low total T-4 or a low free T-4 index, it means you may have hypothyroidism.
Several other medical conditions may cause high or low levels of T-4.
How is this test done?
The test requires a blood sample, which is drawn through a needle from a vein in your arm.
Does this test pose any risks?
Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.
What might affect my test results?
A number of medicines can affect your results. Being pregnant can also affect your results.
How do I get ready for this test?
You don't need to prepare for this test. But be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.