It is less abundant than bound T3 but is the active form of the hormone. Total T3 tests measure both forms of T3. Too much or too little T3 in the blood can indicate a health condition. Many factors influence T3 levels, including age, sex, and some preexisting health conditions , such as liver disease. Pregnancy can also increase total T3 levels. As such, an abnormal level of T3 does not always mean that a person has a thyroid condition. Doctors will typically order T3 tests in conjunction with other thyroid tests to get a better understanding of how the thyroid is functioning.
A doctor may recommend a T3 test for anyone who has the symptoms of a thyroid condition. Thyroid conditions can cause a wide variety of symptoms, including :. A doctor may also recommend the test for people who have a higher risk of thyroid disease. These people include those with:. T3 tests usually require no preparation. However, some medications , such as birth control pills, can affect the results.
Therefore, a person should inform a doctor of any medications that they are taking before undergoing T3 testing. They may need to stop taking them to ensure the accuracy of the test. During a T3 test, a healthcare professional will select a vein to take blood from — either on the inside of the elbow or the back of the hand — and clean the area around it with antiseptic.
They may use an elastic band to restrict the blood flow. Then, they will insert a needle into the vein and draw blood into a syringe. Some people feel a pricking or scratching sensation during blood tests. At this point, if the healthcare professional used an elastic band, they will remove it. When they have collected enough blood, they will remove the needle and apply a cotton ball or bandage. T3 testing generally does not cause side effects, but there is a slight risk of throbbing, soreness, or mild bruising at the injection site.
Some people might also feel lightheaded during or just after the procedure. You can also consider at-home thyroid test kits to measure T3 and other thyroid hormone levels.
High T3 levels can indicate several conditions, including hyperthyroidism and thyrotoxicosis excess circulating hormones. In some cases, they can signify thyroid cancer , but this disease does not often cause abnormal T3 levels. Nodules and lumps that grow on the thyroid can also cause hyperthyroidism. This soon goes away. This test is done to check your thyroid function.
Thyroid function depends on the action of many different hormones, including thyroid-stimulating hormone TSH , T3, and T4. This test helps check the amount of T3 that thyroxin binding globulin TBG is able to bind. TBG is a protein that carries most of the T3 and T4 in the blood. Your provider may recommend a T3RU test if you have signs of a thyroid disorder, including:. Normal value ranges may vary slightly among different laboratories.
Some labs use different measurements or test different samples. Talk to your provider about the meaning of your specific test results. Abnormal results may also be due to an inherited condition of high TBG levels. Usually thyroid function is normal in people with this condition.
There is little risk involved with having your blood taken. Veins and arteries vary in size from one person to another and from one side of the body to the other.
Obtaining a blood sample from some people may be more difficult than from others. Other risks associated with having blood drawn are slight, but may include:.
Evaluation of endocrine function. Test results should be interpreted by a doctor who can take into account your medical history and the results of other tests. One of the main causes of low T3 uptake is a high amount of TBG in the blood. Why is this? Remember that T3 uptake measures the percent of TBG bound to thyroid hormones. If there is an excess amount of TBG, there will be more available to bind, which ultimately decreases the percent of T3 uptake [ 2 ]. Another way T3 uptake can be low is if thyroid hormone levels are low.
There are many different factors that can lead to hypothyroidism. Some of the most common causes include [ 20 ]:. A number of factors may cause a high T3 uptake result. This means most of the TBG in the body would be used up by thyroid hormones, which elevates T3 uptake results [ 2 ]. The most common cause for high thyroid hormone levels is hyperthyroidism, a condition where the thyroid gland releases too much hormone [ 2 ].
The most common causes of hyperthyroidism include [ 32 ]:. What happens if T3 uptake results are abnormal? By itself, abnormal T3 uptake does not generally cause any symptoms. Instead, any symptoms will be from an underlying cause, such as thyroid dysfunction or problems with TBG levels [ 2 ]. Additionally, an abnormal T3 uptake may or may not indicate a thyroid disorder. A total T4 test must also be performed to calculate the free T4 index.
Once the free T4 index is known, your doctor can determine if there is possible hypothyroidism or hyperthyroidism [ 2 ]. The T3 uptake test indirectly measures the amount of TBG that is bound to thyroid hormones. This test, in combination with a total T4 test, can help identify thyroid dysfunction. However, this test has been made obsolete by newer types of tests. Low Thyroid Hormone Levels Hypothyroidism.
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