A T4 (thyroxine) test is a blood test that helps diagnose thyroid conditions. T4 is a thyroid hormone, and too much or too little of it can indicate an issue with your thyroid.
A T4 (thyroxine) test helps diagnose thyroid conditions.
Your thyroid is a small, butterfly-shaped gland located at the front of your lower neck, above your clavicle. It’s a part of your endocrine system. Your thyroid gland makes and releases thyroid hormones into your blood, which then travel to your organs to exert their effect.
Thyroxine, also known as T4, is the major type of hormone your thyroid releases. Too much or too little T4 can indicate thyroid disease.
Healthcare providers test T4 levels using blood tests. Thyroxine (T4) comes in two forms:
Because of this, there are a few different tests that measure T4 levels. A blood test that measures both free and bound T4 is called a total T4 test. Other blood tests measure just free T4. Healthcare providers most often use a free T4 test to assess thyroid function because it’s more accurate than a total T4 test.
Your healthcare provider will most likely also order a TSH (thyroid-stimulating hormone) test alongside a T4 test. TSH is a hormone your pituitary gland makes. It stimulates your thyroid to produce T4 and T3 (triiodothyronine) hormones.
A TSH test is the best way to initially assess thyroid function. In fact, T4 tests more accurately reflect thyroid function when combined with a TSH test. Measuring T4 levels might not be necessary in all thyroid conditions.
Other names for a T4 test include:
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Thyroxine, also known as T4 and tetraiodothyronine, is the main hormone your thyroid gland releases into your bloodstream. Your thyroid also releases small amounts of triiodothyronine (T3). T4 and T3 work together and are commonly referred to as “thyroid hormone.”
Hormones are chemicals that coordinate different functions in your body by carrying messages through your blood to your organs, muscles and other tissues. These signals tell your body what to do and when to do it.
The T4 your thyroid releases is inactive, meaning it doesn’t affect your body’s cells. However, your liver and kidneys convert most of this thyroxine into triiodothyronine (T3), which is an active hormone that impacts the cells in your body.
Together, T4 and T3 play vital roles in regulating various bodily functions, such as:
Healthcare providers prescribe a synthetic form of thyroxine called levothyroxine to treat hypothyroidism (underactive thyroid). They can also prescribe it to treat some kinds of thyroid cancer alongside surgery and other treatments.
Healthcare providers may use T4 tests to assess how well your thyroid is working. Your provider may order a T4 test (in addition to a TSH test) for any of the following reasons:
Again, providers more commonly order free T4 tests than total T4 tests. In certain situations, such as pregnancy, a total T4 test might be necessary rather than a free T4 test.
A phlebotomist usually performs blood draws, including those for a T4 blood test, but any healthcare provider who is trained in drawing blood can perform this task. They then send the samples to a lab where a medical laboratory scientist prepares the samples and performs the tests on machines known as analyzers.
You usually don’t need to do anything special for a T4 test. Depending on the reason for the test, you may need to stop taking certain medications or supplements. In any case, your healthcare provider will give you specific instructions.
Biotin, an over-the-counter supplement, can cause several thyroid tests to appear abnormal when they are in fact normal. It’s recommended to stop biotin-containing supplements 3 to 5 days before any thyroid-related blood draw to avoid this effect.
You can expect to experience the following during a blood test, or blood draw:
The entire procedure usually takes less than five minutes.
After a phlebotomist has collected your blood sample, they’ll send it to a laboratory for testing. Once the test results are back, your provider will share the results with you.
Blood tests are a very common and essential part of medical testing and screening. There’s very little risk to having a T4 blood test. You may have slight tenderness or a bruise at the site of the blood draw, but this usually resolves quickly.
In most cases, you should have your T4 test results within one or two business days, though it could take longer.
Blood test reports, including T4 test reports, usually provide the following information:
Normal levels of free T4 vary based on your age. In general, normal ranges of free T4 for healthy people include:
Normal value ranges for free T4 may vary slightly among different laboratories. Be sure to check your lab report’s reference range on your results. If you have any questions about your results, ask your healthcare provider.
If you have higher-than-normal T4 or free T4 levels, it could indicate thyrotoxicosis. This can result from several situations and conditions, including hyperthyroidism (overactive thyroid), thyroid inflammation (thyroiditis) and taking excessive amounts of thyroid medication.
Thyrotoxicosis speeds up your metabolism, which can be dangerous to your health. Symptoms of thyrotoxicosis include:
If you’re experiencing symptoms of thyrotoxicosis, it’s important to contact your healthcare provider.
Other conditions that could cause elevated total T4 levels with normal free T4 levels include pregnancy and estrogen-containing birth control pills. This is because estrogen levels are high in those two scenarios. Estrogen increases the proteins bound to T4 and causes the total T4 (which is free T4+ binding proteins) to be high.
If you have lower-than-normal T4 levels, it usually indicates hypothyroidism (underactive thyroid). Hypothyroidism has several causes, including certain autoimmune diseases, poor iodine intake in your diet and the use of certain medications.
Hypothyroidism slows down your metabolism. Symptoms include:
If you’re experiencing symptoms of hypothyroidism, it’s important to talk to your healthcare provider.
Free T4 test results are usually accurate. However, certain factors may interfere with the results, including:
Your healthcare provider will consider these factors when interpreting your results.
If your T4 test results are irregular, your provider may order more thyroid tests to help make a diagnosis. These tests may potentially include:
Since thyroid-stimulating hormone (TSH) triggers the production of thyroxine (T4) and temporarily elevated levels of T4 prevent the release of TSH, TSH and T4 levels directly affect each other.
When you get blood tests that check your thyroid function and TSH levels, different levels of each hormone can indicate different conditions.
Having low TSH levels and normal T4 levels is usually considered subclinical hyperthyroidism. “Subclinical” means the condition doesn’t cause any symptoms or symptoms haven’t yet started. Studies estimate that subclinical hyperthyroidism affects up to 16% of the population.
If your blood test report reveals these results, your healthcare provider will likely continue to monitor your levels to see if they change and result in clinical hyperthyroidism.
Having low TSH levels and high T4 levels typically indicate hyperthyroidism. This is because excess T4, due to an issue with your thyroid, is preventing your hypothalamus from releasing TSH.
Having low TSH and low T4 levels may indicate that you have an issue with your pituitary gland, such as a large pituitary adenoma, that’s preventing it from releasing enough TSH to trigger T4 production. This is a less common result combination.
When you have high TSH levels and normal T4 levels, it’s usually considered subclinical hypothyroidism (also called mild thyroid failure). This condition occurs in 3% to 8% of the population.
If your blood test report reveals these results, your healthcare provider will likely continue to monitor your levels to see if they change and result in clinical hypothyroidism.
Having high TSH and low T4 levels usually indicates hypothyroidism, an underactive thyroid due to a primary thyroid problem. This is a lack of T4 production due to an issue with your thyroid that’s causing your pituitary gland to release excess TSH to try to stimulate your thyroid into making more T4.
Having high TSH and high T4 levels may indicate that you have an issue with your pituitary gland that’s causing it to release too much TSH, and thus triggering your thyroid to make excess T4. This is an extremely rare result combination.
A note from Cleveland Clinic
Seeing an abnormal test result can be stressful. Know that thyroid conditions are fairly common, especially hypothyroidism, and are treatable. Your healthcare provider will let you know if you need to undergo further tests to determine the cause of the abnormal T4 level. Don’t be afraid to ask your provider questions or to schedule an appointment to discuss your results further.
Last reviewed by a Cleveland Clinic medical professional on 10/03/2022.
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