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What Can Affect A Thyroid Blood Test

How Is The Test Used


A thyroid panel may be used to evaluate thyroid function and/or help diagnose thyroid disorders.

Typically, the preferred initial test for thyroid disorders is a TSH test. If the TSH level is abnormal, it will usually be followed up with a test for free T4. Sometimes a total T3 or free T3 will also be performed. Often, the laboratory will do this follow-up testing automatically. This is known as reflex testing and it saves the healthcare practitioner time from having to wait for the results of the initial test and then requesting additional testing to confirm or clarify a diagnosis. Reflex tests are typically performed on the original sample that was submitted when the initial test was requested.

As an alternative, a thyroid panel may be requested by your healthcare practitioner. This means that all three tests will be performed at the same time to get a more complete initial picture of thyroid function.

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What Is Being Tested

A thyroid panel is a group of tests that may be ordered together to help evaluate thyroid gland function and to help diagnose thyroid disorders. The tests included in a thyroid panel measure the level of thyroid hormones in the blood.

A thyroid panel usually includes tests for:

  • Free T3 or total T3
  • Although rarely used these days, sometimes a T3 resin uptake test is included. T3RU and T4 can be used to calculate a free thyroxine index . This is another method for evaluating thyroid function. It corrects for changes in certain proteins that can affect total T4 levels.

    The thyroid is a small butterfly-shaped organ that lies flat across the windpipe at the base of the neck that produces thyroid hormones, primarily T4 and some T3. These hormones travel throughout the body and regulate the metabolism by telling the cells in the body how fast to use energy and produce proteins. Most of the hormone produced by the thyroid is T4. This hormone is relatively inactive, but it is converted into the much more active T3 in the liver and other tissues.

    Almost all of the T3 and T4 circulating in the blood is bound to protein. The small portions that are not bound are called “free” and are the biologically active forms of the hormones. Lab tests can measure the amount of free T3 or free T4 or the total T3 or total T4 in the blood.

    The body has an elaborate feedback system to control the amount of T4 and T3 in the blood.

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    How To Test For Thyroid Problems At Home

    How to check your thyroid hormones at home?

    The Everlywell at-home thyroid test can tell you if your thyroid hormone levels suggest hypothyroidism. This test requires only a few drops of blood as a sample. After getting your online test results, you can easily see your levels of thyroid-stimulating hormone , free T3 and T4, and TPO antibodies. High TSH and/or low T3 and T4 can indicate hypothyroidism.

    You can also take our thyroid test at home to check your hormones for indications of hyperthyroidism. Hyperthyroidism means your thyroid gland is overactive: itâs producing too many hormones. Low levels of TSH and/or high T3 and T4 can suggest hyperthyroidism.

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    As medical science advances, we have more tests and biomarkers available to help identify illnesses. Yet overdiagnosis and overtreatment that may occur following abnormal results can cause dangerous adverse effects and costly consequences. Hypothyroidism a lower than normal range of thyroid hormones may be the poster child for this problem because it is such a common condition.

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    What Is A T4 Test

    T4 Tests A Total T4 test measures the bound and free thyroxine hormone in the blood. A Free T4 measures what is not bound and able to freely enter and affect the body tissues.

    What does it mean if T4 levels are abnormal? Importantly, Total T4 levels are affected by medications and medical conditions that change thyroid hormone binding proteins. Estrogen, oral contraceptive pills, pregnancy, liver disease, and hepatitis C virus infection are common causes of increased thyroid hormone binding proteins and will result in a high Total T4. Testosterone or androgens and anabolic steroids are common causes of thyroid hormone binding proteins and will result in a low Total T4.

    In some circumstances, like pregnancy, a person may have normal thyroid function but Total T4 levels outside of the normal reference range. Tests measuring free T4 either a free T4 or free T4 index may more accurately reflect how the thyroid gland is functioning in these circumstances. An endocrinologist can determine when thyroid disease is present in the context of abnormal thyroid binding proteins.

    What Is A T3 Test

    T3 TestsT3 tests measure triiodothyronine levels in the blood. A Total T3 test measures the bound and free fractions of triiodothyronine. Hyperthyroid patients typically have an elevated Total T3 level. T3 tests can be used to support a diagnosis of hyperthyroidism and can determine the severity hyperthyroidism.

    In some thyroid diseases, the proportions of T3 and T4 in the blood change and can provide diagnostic information. A pattern of increased T3 vs T4 is characteristic of Graves disease. On the other hand, medications like steroids and amiodarone, and severe illness can decrease the amount of thyroid hormone the body converts from T4 to T3 resulting in a lower proportion of T3.

    T3 levels fall late in the course of hypothyroidism and therefore are not routinely used to evaluate patients with underactive or surgically absent thyroid glands.

    Measurement of Free T3 is possible, but is often not reliable and therefore may not be helpful.

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    What Are Normal Thyroid Hormone Levels

    The thyroid itself is regulated by another gland that is located in the brain, called the pituitary. In turn, the pituitary is regulated in part by the thyroid and by another gland called the hypothalamus.

    The hypothalamus releases a hormone called thyrotropin releasing hormone , which sends a signal to the pituitary to release thyroid stimulating hormone . In turn, TSH sends a signal to the thyroid to release thyroid hormones. If a disruption occurs at any of these levels, a defect in thyroid hormone production may result in a deficiency of thyroid hormone .

    Hypothalamus – TRH

    Thyroid – T4 and T3

    The rate of thyroid hormone production is controlled by the pituitary gland. If there is an insufficient amount of thyroid hormone circulating in the body to allow for normal functioning, the release of TSH is increased by the pituitary gland in an attempt to stimulate more thyroid hormone production. In contrast, when there is an excessive amount of circulating thyroid hormone, TSH levels fall as the pituitary attempts to decrease the production of thyroid hormone.

    • In persons with hypothyroidism , there is a continuously decreased level of circulating thyroid hormones.
    • In persons with hyperthyroidism , there is a continuously elevated level of circulating thyroid hormones.

    Thyroid Testing: How It Works

    Thyroid #2 Essential Blood Test Panel from Walk-in Lab

    Thyroid tests use a blood sample to measure various hormone levels. Thyroid-stimulating hormone , for example, is often measured to indirectly check if your thyroid gland is producing enough thyroid hormones. TSH levels are usually higher than normal if you have an underactive thyroid .

    Thyroid function tests can also measure thyroid hormones like thyroxine to provide a better understanding of how low â or how high â your thyroid hormone levels are.

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    Magnetic Resonance Imaging Scan

    MRI scans use magnets instead of radiation to create detailed cross-sectional images of your body. MRI can be used to look for cancer in the thyroid, or cancer that has spread to nearby or distant parts of the body. But ultrasound is usually the first choice for looking at the thyroid. MRI can provide very detailed images of soft tissues such as the thyroid gland. MRI scans are also very helpful in looking at the brain and spinal cord.

    What Are We Testing

    This is the first question. Some doctors only test TSH and gauge thyroid function from that. For me, TSH is a start, but it does not give me the whole picture. I like to test what the clinicians in the study used: free T3 and free T4 in addition to TSH.

    That said, many doctors order a thyroid panel consisting of TSH with total T4 and T3 uptake , and a calculated T7 index. This gives some information to interpret the TSH value, but it is not complete.

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    What Is A Tsh Test

    Thyroid testsBlood tests to measure thyroid hormones are readily available and widely used. Not all thyroid tests are useful in all situations.

    TSH TestThe best way to initially test thyroid function is to measure the TSH level in a blood sample. Changes in TSH can serve as an “early warning system” often occurring before the actual level of thyroid hormones in the body becomes too high or too low.

    A high TSH level indicates that the thyroid gland is not making enough thyroid hormone . On the other hand, a low TSH level usually indicates that the thyroid is producing too much thyroid hormone . Occasionally, a low TSH may result from an abnormality in the pituitary gland, which prevents it from making enough TSH to stimulate the thyroid . In most healthy individuals, a normal TSH value means that the thyroid is functioning properly.

    What Is A Normal Thyroid Level

    Thyroid Testing

    Tests often used to assess thyroid hormone status include TSH and FT4 tests. The normal value for a laboratory test is determined by measuring the hormone in a large population of healthy individuals and finding the normal reference range. Normal ranges for thyroid tests may vary slightly among different laboratories, and typical ranges for common tests are given below.

    TSH normal values are 0.5 to 5.0 mIU/L. Pregnancy, a history of thyroid cancer, history of pituitary gland disease, and older age are some situations when TSH is optimally maintained in different range as guided by an endocrinologist.

    FT4 normal values are 0.7 to 1.9ng/dL. Individuals taking medications that modify thyroid hormone metabolism and those with a history of thyroid cancer or pituitary disease may be optimally managed with a different normal FT4 range.

    Total T4 and Total T3 levels measure bound and free thyroid hormone in the blood. These levels are influenced by many factors that affect protein levels in the body, including medications, sex hormones, and liver disease. A normal Total T4 level in adults ranges from 5.0 to 12.0g/dL. A normal Total T3 level in adults ranges from 80-220 ng/dL.

    Free T3 assays are often unreliable and not routinely used to assess thyroid function.

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    Which Drugs Can Lower Tsh Without Inducing True Thyroid Dysfunction

    • Glucocorticoids in high doses during initial treatment in contrast, prolonged exposure to glucocorticoid therapy or endogenous hypercortisolism do not result in thyroid dysfunction

    • Dopamine or dobutamine

    These drugs do not generally cause clinically significant central hypothyroidism, and their suppressant effect on TSH production is transient. An exception is bexarotene, in which the associated central hypothyroidism that is occasionally seen normalizes after the discontinuation of the medication.

    What Do My Thyroid Test Results Mean

    The two most common types of thyroid function tests are the T4 and TSH test. Typically, these two tests are ordered together.

    The T4 testmore commonly known as the thyroxine testis used to determine if your thyroid is overactive. If this is the case, you will be diagnosed with hyperthyroidism.

    Symptoms of hyperthyroidism include unplanned weight loss, tremors, diarrhea and anxiety.

    The TSH test will reveal how much thyroid-stimulating hormone is in your blood. Normal levels range from 0.4 to 4.0 milli-international units of hormone per liter of blood.

    If youve been diagnosed with hyperthyroidism and your TSH levels are above 2.0 mIU/L, you are at a heightened risk of developing hypothyroidism, which can result in weight gain, fatigue, depression and weak hair and nails.

    If your doctor believes you have hyperthyroidism, he or she may order a T3 test to check for levels of the hormone triiodothyronine.

    This hormone level should be within 100-200 nanograms of hormone per deciliter of blood. If your levels are abnormally high, it is possible that you are suffering from an autoimmune disorder called Graves disease.

    The final thyroid function test your doctor may order is the T3 resin uptake test. Also known as a T3RU, this blood test will measure the binding capacity of a hormone called thyroxin-binding globulin.

    Your doctors office should reach out to you within a matter of days to walk you through the results of your thyroid test.

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    Medications Affecting The Thyroid Test

    Common preparations, over the counter drugs, and minerals may affect your thyroid test and your thyroid health. Common medications that affect thyroid and antibody blood tests are:

    • lithium: thyroid antibodies have been found with intake of lithium. Lithium is concentrated in the gland and interferes with thyroid hormone synthesis/release and an increase in thyrotropin levels.
    • estrogen and estrogen-containing birth control pills: interferes with total T4 and total T3 measurements.
    • heroin and methadone hydrochloride: interferes with total T4 and total T3 measurements.
    • dopamine agonists: suppress thyrotropin levels to lower-than-normal values. Thyrotropin is undetectable in patients with true hyperthyroidism.
    • nonsteroidal anti-inflammatory drugs: displace thyroid hormones from thyroid-binding globulin, increase FT4 and FT3 concentrations and depress thyrotropin levels.
    • androgens: interferes with total T4 and total T3 measurements.
    • iodides: are hidden in many preprations and non-prescription drugs can induce iodide hypothyroidism and goiter.
    • anabolic steroids: interferes with total T4 and total T3 measurements.
    • nicotinic acid: interferes with total T4 and total T3 measurements.
    • glucocorticoids: interferes with total T4 and total T3 measurements.
    • beta blockers and corticosteroids: minimally inhibit the peripheral conversion of T4 to T3.

    What To Do In Preparation For A Thyroid Blood Test

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    While it would seem to make sense to abstain from any not-so-great habits before having a thyroid blood test, it is probably best to assess your thyroid under normal conditions for you. If you drink alcohol each day, it is best not to alter that pattern if you plan to continue drinking this amount after your test.

    You want to stick with your regular habits because you need to understand how your thyroid functions in your everyday life. Of course, you also need to let your doctor know how much you drink on average each day or week so they can take that into account when reviewing your labs.

    Aside from alcohol, several other factors can also skew your thyroid lab results. For example, the time of day you take your labs may have subtle influences on your thyroid labs, which can be problematic if you have subclinical hypothyroidism and are working with your thyroid doctor to decide if you need treatment.

    Also, women who are not menopausal may experience subtle shifts in their thyroid hormones depending on where they are in their cycle.

    To avoid discrepancies between current and past lab results, it is advisable that each time you test your thyroid, test around the same time of day, in the same phase of your menstrual cycle, and follow your usual daily habits.

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    How To Test Your Thyroid Function

    You can test your thyroid function using a blood sample. The two main ways to collect a blood sample are:

    • a venous blood test a trained professional will use a needle to puncture a vein, usually in your arm, to collect a blood sample.
    • a finger-prick blood test using a lancet, you can prick your own finger and collect a small blood sample.

    Why Do I Need A Tsh Test

    You may need a TSH test if you have symptoms of too much thyroid hormone in your blood , or too little thyroid hormone .

    Symptoms of hyperthyroidism, also known as overactive thyroid, include:

    Symptoms of hypothyroidism, also known as underactive thyroid, include:

    • Weight gain
    • Low tolerance for cold temperatures
    • Irregular menstrual periods
    • Constipation

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    Biotin Hair Products: Can They Affect Thyroid Tests

    I was just wondering if using biotin shampoo and conditioner can affect thyroid tests ?

    Welcome to Mayo Connect, a community of people living with a wide variety of diseases and conditions, who support each other along the way. We try to be informed medical consumers, and our own best advocates in our care. We are not medical professionals, so not able to provide medical advice. We can tell you what has worked for us and provide you with information and conversation about your questions.

    May I ask what led you to Mayo Connect with such an interesting question? I love hearing these things, as they make me think and read. Your question led me to this article about biotin supplements and thyroid tests: which of course doesn’t address conditioners or other things you put on your body.

    Here are some things to consider. The amount of biotin that is in shampoo or conditioner is really tiny compared to the 5-10mg of biotin that interferes with tests. What is put on hair only enters your body through scalp or skin, not the hair shaft so it’s really tiny in amount. Finally, biotin placed on your hair will not promote growth, as it does not enter your body through the hair shaft, which is essentially dead. So if your body is deficient in biotin, to promote hair growth you need to get it inside you, either through a healthy diet or a supplement.


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