If your doctor is monitoring your thyroid, he will measure the levels of certain thyroid hormones found in your blood. These hormones include thyroxine and triiodothyronine, which blood level results are used to determine whether you have a thyroid condition, like hyperthyroidism or hypothyroidism. The T4 blood test will measure thyroxine, while a T3 total blood test will measure triiodothyronine. With a T3 blood test, different measurements are made from the blood sample.
Function
The T3 total blood test has several different functions. Initially, it will be performed to determine if the person has thyroid disease. Once thyroid disease is diagnosed, medications will be prescribed to control the levels of thyroxine and triiodothyronine found in the body. The T4 total blood test will let the physician know how well the medications are working.
Features
The T3 total blood test will measure the complete amount of T3 found in the patient's blood, including the triiodothyroxine bound to carrier proteins as well as triiodothyroxine that circulates unbound. In a normal range blood test, the T4 number will be higher than the T3. However, if the person suffers from hyperthyroidism, the T3 is likely to be higher.
Considerations
A T3 blood test is rarely performed alone and will almost always be performed in conjunction with other thyroid hormone tests. Your physician will likely order the T4 blood test and a TSH blood test as well with the T3. If the TSH and T4 come back out of the normal range, it indicates that the person has hypothyroidism or hyperthyroidism.
Identification
Your lab and doctor will provide you with a normal range for your T3 total blood test results. According to the National Library of Medicine, normal values fall in the 100 to 200 nanograms per decileter range. Different normal ranges will be given for babies, children and pregnant women.
Warning
Certain medications can affect the results of a T3 total blood test, so it is important to let your doctor know if you are taking any drugs. T3 levels can increase with birth control, estrogen and methadone use. T3 measurements can decrease when a person uses steroids, lithium or androgen.
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