Does this test have other names?
Free thyroxine test
What is this test?
This test measures the levels of free T4, or free thyroxine, in your blood. A free T4 test is used to find out how well your thyroid is working.
T4 is one of two hormones produced by the thyroid, a butterfly-shaped gland in your neck. Some T4 in your blood is bound to proteins, and some T4 circulates freely, or unbound to proteins. The free T4 test measures unbound T4.
The other thyroid hormone is triiodothyronine, or T3.
These hormones help regulate your body's metabolism. They go into action when prompted by thyroid stimulating hormone (TSH), which is released by your pituitary gland.
Why do I need this test?
You may need this test to find out whether you have a thyroid-related condition such as hyperthyroidism, which means an overactive thyroid, or hypothyroidism, which means an underactive thyroid.
Each condition has many different symptoms. If you have hyperthyroidism, you may often feel anxious and irritable, have trouble sleeping, and have an irregular heart rhythm. You may also feel quite tired and notice that you're losing weight.
If you have hypothyroidism, you may notice weight gain, even if you aren't overeating. You may also be more sensitive to cold, have low energy, and have dry skin and hair.
What other tests might I have along with this test?
You may have tests to measure T3 and TSH. These hormones also play key roles in thyroid health. You may also have a blood test to measure the level of certain antithyroid antibodies in your blood to get a more accurate diagnosis.
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.
The normal range for free T4 is 0.8 to 2.8 nanograms per deciliter (ng/dL). A level of free T4 that is higher than normal could mean you have an overactive thyroid. Conditions associated with hyperthyroidism include Graves' disease, an autoimmune disorder.
Abnormally low free T4 levels may signal hypothyroidism. This means your thyroid is not making enough hormones. An underlying condition, such as Hashimoto's disease, another autoimmune disorder, could be the cause of an underactive thyroid.
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?
Certain medications, such as phenobarbital, can affect your free T4 levels. Severe chronic illnesses, such as chronic renal failure and cirrhosis of the liver, can also affect the test.
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.