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Free T4

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T4 is one of the hormones made by the thyroid, the butterfly-shaped gland found in front of the windpipe. T4 and triiodothyronine (T3) 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 T4 in the blood is bound to thyroxine-binding globulin. Less than one percent of the T4 is free. The free T4 blood test is used to help determine what may be causing an abnormal thyroid-stimulating hormone (TSH) test. Measurements of free T4 levels are not affected by certain protein levels, so they remain more constant. Measuring free T4 gives a more accurate picture of the true concentration of T4 levels in the blood.

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 free T4 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.