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This test measures the amount of free thyroxine, or T4, in the blood. T4 is one of two major hormones produced by the thyroid gland; the other is called triiodothyronine, or T3. Most of the thyroxine (T4) in the blood is attached to a protein called thyroxine-binding globulin. Less than 1% of the T4 is unattached. A total T4 blood test measures both bound and free thyroxine. Free thyroxine affects tissue function in the body, but bound thyroxine does not.