Total T4 concentration, thyroxine screen, free T4 concentration
This is a blood test to measure your level of the hormone thyroxine, or T4.
T4 is made in your thyroid, a butterfly-shaped gland located near the base of your throat above your collarbones. In addition to T4, your thyroid also makes T3. Both hormones affect your energy level, mood, weight, and other important elements of your health.
T4 is found in the body in two forms: free T4 and bound T4. Free T4 travels into body tissues that use T4. Bound T4 attaches to proteins that prevent it from entering these tissues. More than 99 percent of T4 is bound.
This test can show your doctor whether your thyroid gland is overactive, a condition called hyperthyroidism, or underactive, a condition called hypothyroidism.
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:
Low tolerance for cold
Slower heart rate
Shortness of breath
Loss of consciousness (rare)
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
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 T4 – both free and bound – varies by laboratory, but the usual range is 4.6 to 11.2 mcg/dL.
Free T4 is usually measured two ways:
Free T4, with normal ranges determined by the testing method the lab uses
Free T4 index, a formula that includes total T4 and a measurement called thyroid hormone-binding index. The normal range for the free T4 index is 1.1 to 4.3 mcg/dL.
If your results show high total T4 or a high free T4 index, it means you may have hyperthyroidism. If your results show low total T4 or a low free T4 index, it means you may have hypothyroidism.
Several other medical conditions may cause high or low levels of T4.
The test requires a blood sample, which is drawn through a needle from a vein in your arm.
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.
A number of medications can affect your results. Being pregnant can also affect your results.
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.