This hormone plays a role in several of your body’s functions, including growth and metabolism.
Your thyroid produces a hormone called thyroxine, which is known as T4.
Part of your T4 exists as free T4. This means that it has not joined the protein in your blood.
This is the type available for your body and tissues. However, most of the T4 in the bloodstream is bound to proteins.
Because T4 exists in two forms in your body, there are two types of T4 tests:
- A real test of T4.
- An examination of free T4.
A total T4 test measures the T4 bound to the protein and any free T4.
A free T4 trial: measures only free T4 in your blood.
Because free T4 is available to your body, a total T4 test is often preferred.
Why do doctors perform a T4 test?
Your doctor may order a T4 test if a thyroid-stimulating hormone (TSH) test returns with abnormal results.
A T4 test will help your doctor determine what type of problem affects your thyroid. Some disorders that affect thyroid function include:
- Hyperthyroidism or overactive thyroid.
- Hypothyroidism or underactive thyroid.
- Hypopituitarism or hypoactive pituitary gland
Your doctor may suspect that one of these conditions is present if you have symptoms such as:
- Eye problems, such as dryness, irritation, swelling, and bulging.
- Dryness or swelling of the skin
- Hair loss.
- Hand tremors.
- Changes in heart rate
- Changes in blood pressure
You may also experience more general symptoms such as:
- Weight changes
- Difficulty sleeping or insomnia.
- Fatigue and weakness
- Intolerance to the cold.
- Sensitivity to light.
- Menstrual irregularity
Sometimes, your doctor may also order other thyroid tests such as T3 or TSH. TSH, or thyroid stimulating hormone, comes from the pituitary gland and stimulates the thyroid to release T3 and T4.
Performing one or both of these tests can help your doctor better understand your thyroid problem.
In some cases, your doctor may perform one or more of these tests to help evaluate if a known thyroid problem is improving.
Preparation for a T4 test
Several medications can interfere with your T4 levels, especially total T4, so telling your doctor what medications you are taking is essential.
You may need to stop taking certain medications temporarily before the test to help ensure accurate results.
It is also important to tell your doctor if you may be pregnant. Medications that can affect your T4 levels include:
- Medicines contain hormones, such as androgens, estrogens, and birth control pills.
- Drugs designed to affect your thyroid or treat thyroid conditions.
- Medications designed to treat cancer
These are not the only drugs that can affect your results. Tell your doctor about all medications and any herbal supplements you use.
Procedure for a T4 test
Your healthcare provider will collect your blood in a tube or vial and send it to a lab for examination.
Typical results for the total T4 test in adults generally range from 4.5 to 11.2 micrograms per deciliter (μg / dL). The results for children vary according to age. So talk to your doctor about the normal ranges expected for your child.
There may also be some variation between laboratories. The typical results in adults for the free T4 test generally vary from 0.9 to 2.4 nanograms per deciliter (ng / dL).
Like total T4 in adults, free T4 also varies in children according to age.
As with any test result, if you are outside the expected range, talk to your doctor about the effects they may have on your health condition.
Because T4 is not the only hormone involved in thyroid function, a typical result in this test could mean that there is a thyroid problem.
For example, the results of T4 could fall in a normal range, but the results of T3 could be abnormal. This may be particularly true in cases of an overactive thyroid.
What do the abnormal results of the T4 test mean?
An abnormal result on the T4 test alone may not provide enough information to your doctor to completely understand or diagnose your condition.
They may need to consider the results of T3 and TSH levels to obtain a complete picture. Pregnancy can also affect your T4 levels. If your T4 levels are abnormal but you are pregnant, your doctor may order more tests.
Elevated levels of T4 may indicate hyperthyroidism. They can also indicate thyroid problems, such as thyroiditis or toxic multinodular goiter.
Abnormal results can also be caused by:
- High levels of protein in the blood.
- Too much iodine
- Too much thyroid replacement medication.
- A trophoblastic disease is a group of rare tumors related to pregnancy.
- Germ cell tumors. Too much iodine can raise your T4 levels. Because x-ray dyes may include iodine, a recent x-ray with dye may also increase the results of the T4 test.
Abnormally low levels of T4 may indicate:
- Dietary problems, such as fasting, malnutrition, or an iodine deficiency.
- Medications that affect protein levels.
Risks of a T4 test
A T4 test does not have specific risks. Risks include those present each time blood is drawn. In rare cases, you may experience complications, such as an inflamed vein, infection, or excessive bleeding.
You may feel pain or discomfort during the extraction of blood. You may also bleed slightly after removing the needle and 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 is a low-risk procedure often used with other blood tests, including T3 and TSH tests.