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Triiodothyronine (T3)

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T3 is one of the hormones made by the thyroid, the butterfly-shaped gland found in front of the windpipe. T3 and thyroxine (T4) are essentially the same, although T3 more greatly affects how the body uses energy than T4. The main difference is that T4 is the major storage form of thyroid hormone. It restores the T3 level.

Both T3 and T4 are needed for normal brain development, especially during the first three years of life. These hormones help children grow and develop normally. In adults the hormones regulate metabolism: how the body uses energy. According to the United States Preventive Services Task Force, all newborns should be tested for congenital hypothyroidism.

Most of the T3 in the blood is bound to thyroxine-binding globulin. Less than one percent of the T3 is free. The T3 blood test assesses both free and bound T3 to help determine what may be causing an abnormal thyroid-stimulating hormone (TSH) test.

The test is also ordered for people who have been diagnosed with a thyroid diseases or disorder to check the efficacy of the thyroid treatment and determine if dosages should be changed. The T3 test can be a screen for newborns to see if their thyroid gland functions are normal. Catching congenital hypothyroidism early can prevent abnormal growth and development and intellectual disability.