Total T4 concentration, thyroxine screen, free T4 concentration
This test measures the level of the hormone thyroxine (T4) in your blood. The hormone is made by your thyroid gland. The thyroid is a butterfly-shaped gland near the base of your throat, above your collarbones.
The thyroid gland makes T4 in response to thyroid-stimulating hormone (TSH). TSH is made by the pituitary gland in your brain.
T4 is found in the body in 2 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% of T4 is bound. Because T4 is converted into another thyroid hormone called T3 (triiodothyronine), free T4 is the more important hormone to measure. Any changes show up in T4 first.
T3 and T4 help to control how your body stores and uses energy (metabolism). The thyroid hormones also help control many of your body's other processes. These include:
Nervous system function
How easily you gain weight
Moisture in the skin
This test can show your healthcare provider if your thyroid gland is overactive (hyperthyroidism) or underactive (hypothyroidism).
You may need this test if you have symptoms of thyroid problems.
Symptoms of hyperthyroidism, or too much thyroid activity, include:
Anxiety and mood swings
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
Thinning of hair
High blood sugar
Shortness of breath
Symptoms of hypothyroidism, or less than normal thyroid activity, include:
Low tolerance for cold
Slower heart rate
Loss of consciousness (rare)
High blood pressure (hypertension) or worsening hypertension
Carpal tunnel syndrome
You may need other tests to measure thyroid-related substances. These include:
TSH, T3, and free T3
Imaging scans such as ultrasound of the thyroid gland
Radioactive iodine uptake scan of the thyroid (RAIU scan)
Thyroglobulin, which is used to make and store thyroid hormones
TSH receptor-stimulator antibodies, which are used to diagnose Graves disease
Thyroid antiperoxidase antibodies and thyroglobulin antibodies, which are used to diagnose Hashimoto's thyroiditis
Test results may vary depending on your age, gender, health history, the method used for the test, and other things. Your test results may not mean you have a problem. Ask your healthcare provider what your test results mean for you.
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 2 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 T 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 health conditions may cause high or low levels of T.
The test is done with a blood sample. A needle is used to draw blood from a vein in your arm or hand.
Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be sore.
A number of medicines can affect your results. Being pregnant can also affect your results.
You don't need to prepare for this test. But be sure your healthcare provider knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any Illegal drugs you may use.