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Topic Overview The thyroid gland uses iodine from food to make two thyroid hormones: triiodothyronine T3 and thyroxine T4. When the hypothalamus and pituitary are working normally, they sense when: Thyroid hormone levels are low, so they secrete more TRH and TSH, which stimulates the thyroid to make more hormones. Thyroid hormone levels are too high, so they secrete less TRH and TSH, which reduces hormone production by the thyroid. Disease or tumors of the pituitary gland can affect this process.
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Either test may be used to check T3 levels. If T3 levels are not normal, it can be a sign of thyroid disease. A T3 test is most often used to diagnose hyperthyroidism , a condition in which the body makes too much thyroid hormone. T3 tests are frequently ordered with T4 and TSH thyroid stimulating hormone tests. A T3 test may also be used to monitor treatment for thyroid disease. A health care professional will take a blood sample from a vein in your arm, using a small needle.
After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes. You don't need any special preparations for a T3 blood test. Your health care provider will let you know if you need to stop taking any medicines before your test.
Certain medicines can raise or lower T3 levels. There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.
If your results show high total T3 levels or high free T3 levels, it may mean you have hyperthyroidism. Low T3 levels may mean you have hypothyroidism , a condition in which your body doesn't make enough thyroid hormone.
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