Your thyroid produces a hormone called 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 hasn’t bonded to protein in your blood. Most of the T4 in your body does bond with protein.
There are two kinds of T4 tests: a total T4 test and a free T4 test. A total T4 test measures the T4 that has bonded to protein and the free T4. A free T4 test measures only the free T4 in your blood.
Your doctor may order a T4 test if a thyroid-stimulating hormone (TSH) test has come back with abnormal results. A T4 test will help your doctor determine what is 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 may suspect that one of these thyroid conditions is present if you have symptoms such as:
- eye issues, such as dryness, irritation, puffiness, and bulging
- skin dryness or puffiness
- hair loss
- hand tremors
- increased heart rate
You may 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 may 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 may perform one or more of these tests to help assess whether a known thyroid issue is improving.
A number of 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 that contain 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 micrograms per deciliter.
Because T4 isn’t 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.
An abnormal result on the T4 test may not give your doctor enough information to fully understand or diagnose your condition. They may also need to consider the results of tests for T3 and TSH levels.
Pregnancy can affect your T4 levels. If your T4 levels are abnormal but you’re pregnant, your doctor may order further testing. For example, the levels of T3 in your blood may help your doctor understand your situation.
Elevated T4 levels may indicate hyperthyroidism. They may also indicate other thyroid problems, such as thyroiditis or toxic multinodular goiter.
Other abnormal results may indicate:
- 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
Too much iodine can raise your T4 levels. Because X-ray dye includes iodine, a recent X-ray involving dye may also raise your T4 test results.
Abnormally low levels of T4 may indicate:
- dietary issues, such as fasting, malnutrition, or an iodine deficiency
- medications that affect protein levels
A T4 test has no specific risks. Risks include those present whenever you have your blood drawn.
In rare cases, you may experience a serious complication, such as an inflamed vein, an infection, or excessive bleeding.
More commonly, you may feel pain or discomfort during the blood draw. You may also bleed slightly after the needle is removed, and you may develop a small bruise around the puncture site.
A T4 test is a blood test that helps your doctor identify problems with your thyroid. It’s a low-risk procedure that’s often used with other blood tests, including a T3 test and a TSH test. If you’re preparing for a T4 test, make sure to tell your doctor about any medications you’re taking. This will ensure accurate test results.