Your thyroid produces a hormone, thyroxine, which is known as T4. This hormone plays a role in several of your body’s functions, including growth and metabolism.
Some of your T4 is called free T4. This means that it has not bonded to protein in your blood. Most of the T4 in your body does bond with protein.
There are two kinds of T4 tests. The first, known as a total T4 test, measures both kinds of T4 (the T4 that has bonded to protein and the free T4) in your blood. The second, a free T4 test, measures only the free T4 in your blood.
Your doctor will often order a T4 if a thyroid-stimulating hormone (TSH) test has come back with abnormal results and your doctor wants further insight into what could be wrong with your thyroid. Some thyroid disorders include:
- hyperthyroidism (overactive thyroid)
- hypothyroidism (underactive thyroid)
- hypopituitarism (underactive pituitary gland)
- thyrotoxic periodic paralysis (occasional muscle weakness caused by high levels of thyroid hormone)
Your doctor might suspect that one of these thyroid conditions is present if you have symptoms such as:
- eye issues, such as dryness, irritation, puffiness, bulging
- skin dryness or puffiness
- hair loss
- hand tremors
- increased heart rate
You might also experience more general symptoms such as:
- weight changes
- difficulty sleeping or insomnia
- fatigue and weakness
- intolerance to cold
- sensitivity to light
- menstrual irregularity
Sometimes, your doctor might also order a test for T3 and/or TSH. The TSH, or thyroid-stimulating hormone, stimulates your thyroid to release both T3 and T4. Performing one or both of these other tests may help your doctor gain a better understanding of your thyroid problem.
In some cases, your doctor might perform one or more of these tests to help assess whether a known thyroid issue is improving.
Quite a few drugs can interfere with your T4 levels, so it’s important to tell your doctor what medications you’re taking. You may need to stop taking certain drugs temporarily before the test to help ensure accurate results.
Medications that can affect your T4 levels include:
- drugs including hormones, such as androgen, estrogen, birth control pills
- drugs designed to affect your thyroid or treat thyroid conditions
- some drugs designed to treat cancer
These are not the only drugs that can affect your results. Make sure to tell your doctor about every medication you use.
Your healthcare provider will collect your blood into a tube or vial and send it to a lab for testing.
Normal results are generally from 4.5 to 11.2 mcg/dL (micrograms per deciliter).
Because T4 is not the only hormone involved in thyroid function, a normal result on this test may not mean that you do not have a thyroid problem. For example, your T4 results could fall into a normal range but your T3 results could be elevated.
Successfully interpreting an abnormally high or low result in the T4 test often involves incorporating the results of tests for T3 and thyroid stimulating hormone (TSH) levels. An abnormal result on this test alone may not give your doctor enough information to fully understand or diagnose your condition.
Pregnancy can affect your T4 levels. If your T4 levels are abnormal but you are pregnant, your doctor may order further testing. For example, the levels of T3 in your blood might help shed light on the situation.
Elevated T4 levels might indicate:
- hyperthyroidism, such as iodine-induced hyperthyroidism or Graves’ disease (an autoimmune disease that causes hyperthyroidism)
- other thyroid problems, such as thyroiditis (inflammation of the thyroid gland) or toxic multinodular goiter (an enlarged thyroid gland with round growths that produce high levels of thyroid hormone)
Other issues include:
- high levels of protein in the blood
- too much iodine
- too much thyroid replacement medication
- trophoblastic disease, a group of rare pregnancy-related tumors
- germ cell tumors, types of tumors that occur in specialized cells found in the ovaries or testes
Too much iodine can raise your T4 levels, and because X-ray dye includes iodine, a recent X-ray involving dye may also raise your T4 test results.
Abnormally low levels of T4, on the other hand, may indicate the following:
- dietary issues, such as fasting, malnutrition, too little iodine
- medications that affect protein levels
The T4 test has no specific risks. Risks include those present whenever you have your blood drawn.
More commonly, you may experience pain or discomfort during the blood draw. You might also bleed slightly after the needle is removed, and you might develop a small bruise around the puncture site.